yugabyte#2101: Fixed race between Tablet::Flush and Tablet::Shutdown

Summary:
```
WARNING: ThreadSanitizer: data race (pid=11311)
1762	   Read of size 8 at 0x7b74000cfb58 by thread T155:
1763	     #0 std::__1::unique_ptr<rocksdb::DB, std::__1::default_delete<rocksdb::DB> >::operator bool() const /opt/yb-build/thirdparty/yugabyte-db-thirdparty-v20200829090443-f431681041-centos/installed/tsan/libcxx/include/c++/v1/memory:2619:19 (libtablet.so+0x211067)
1764	     #1 yb::tablet::Tablet::Flush(yb::tablet::FlushMode, yb::tablet::FlushFlags, long) src/yb/tablet/tablet.cc:1847 (libtablet.so+0x211067)
1765	     #2 yb::tserver::MiniTabletServer::FlushTablets(yb::tablet::FlushMode, yb::tablet::FlushFlags)::$_2::operator()(yb::tablet::TabletPeer*) const src/yb/tserver/mini_tablet_server.cc:200:35 (libtserver.so+0x1a0997)
...
1771	     #8 yb::tserver::MiniTabletServer::FlushTablets(yb::tablet::FlushMode, yb::tablet::FlushFlags) src/yb/tserver/mini_tablet_server.cc:196:10 (libtserver.so+0x19f4e8)
1772	     #9 yb::MiniCluster::FlushTablets(yb::tablet::FlushMode, yb::tablet::FlushFlags) src/yb/integration-tests/mini_cluster.cc:369:5 (libintegration-tests.so+0x10cf88)
1773	     #10 yb::client::QLStressTest_LongRemoteBootstrap_Test::TestBody()::$_8::operator()() const src/yb/client/ql-stress-test.cc:972:7 (ql-stress-test+0x4f2f1d)

Previous write of size 8 at 0x7b74000cfb58 by thread T49 (mutexes: write M263877791325157728):
1780	     #0 std::__1::unique_ptr<rocksdb::DB, std::__1::default_delete<rocksdb::DB> >::reset(rocksdb::DB*) /opt/yb-build/thirdparty/yugabyte-db-thirdparty-v20200829090443-f431681041-centos/installed/tsan/libcxx/include/c++/v1/memory:2632:20 (libtablet.so+0x209c9c)
1781	     #1 yb::tablet::ResetRocksDB(bool, rocksdb::Options const&, std::__1::unique_ptr<rocksdb::DB, std::__1::default_delete<rocksdb::DB> >*) src/yb/tablet/tablet.cc:950 (libtablet.so+0x209c9c)
1782	     #2 yb::tablet::Tablet::ResetRocksDBs(yb::StronglyTypedBool<yb::tablet::Destroy_Tag>, yb::StronglyTypedBool<yb::tablet::DisableFlushOnShutdown_Tag>) src/yb/tablet/tablet.cc:967:27 (libtablet.so+0x209b3b)
1783	     #3 yb::tablet::Tablet::CompleteShutdown(yb::StronglyTypedBool<yb::tablet::IsDropTable_Tag>) src/yb/tablet/tablet.cc:931:3 (libtablet.so+0x205bac)
1784	     #4 yb::tablet::TabletPeer::CompleteShutdown(yb::StronglyTypedBool<yb::tablet::IsDropTable_Tag>) src/yb/tablet/tablet_peer.cc:475:14 (libtablet.so+0x287f71)
1785	     #5 yb::tablet::TabletPeer::Shutdown(yb::StronglyTypedBool<yb::tablet::IsDropTable_Tag>) src/yb/tablet/tablet_peer.cc:529:5 (libtablet.so+0x28896d)
1786	     #6 yb::tserver::TSTabletManager::DeleteTablet(string const&, yb::tablet::TabletDataState, boost::optional<long> const&, boost::optional<yb::tserver::TabletServerErrorPB_Code>*) src/yb/tserver/ts_tablet_manager.cc:1297:16 (libtserver.so+0x22d59e)
1787	     #7 yb::tserver::TabletServiceAdminImpl::DeleteTablet(yb::tserver::DeleteTabletRequestPB const*, yb::tserver::DeleteTabletResponsePB*, yb::rpc::RpcContext) src/yb/tserver/tablet_service.cc:1158:41 (libtserver.so+0x1e1de7)
1788	     #8 yb::tserver::TabletServerAdminServiceIf::Handle(std::__1::shared_ptr<yb::rpc::InboundCall>) src/yb/tserver/tserver_admin.service.cc:130:7 (libtserver_admin_proto.so+0x73cb0)
1789	     #9 yb::rpc::ServicePoolImpl::Handle(std::__1::shared_ptr<yb::rpc::InboundCall>) src/yb/rpc/service_pool.cc:262:19 (libyrpc.so+0x20aa57)
1790	     #10 yb::rpc::InboundCall::InboundCallTask::Run() src/yb/rpc/inbound_call.cc:212:13 (libyrpc.so+0x1745ee)
```

Added missing `ScopedRWOperation` into `Tablet::Flush` to avoid destroying tablets during flush.

Test Plan: `ybd --remote --dltp tsan --cxx-test client_ql-stress-test --gtest_filter QLStressTest.LongRemoteBootstrap -n 500 -- -p 1`

Reviewers: bogdan, mikhail

Reviewed By: mikhail

Subscribers: zyu, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D9541

Author: ttyusupov

src/yb/tablet/tablet.cc

@@ -1836,6 +1836,8 @@ void Tablet::AcquireLocksAndPerformDocOperations(std::unique_ptr<WriteOperation>
 Status Tablet::Flush(FlushMode mode, FlushFlags flags, int64_t ignore_if_flushed_after_tick) {
   TRACE_EVENT0("tablet", "Tablet::Flush");
 
+  ScopedRWOperation pending_op(&pending_op_counter_);
+
   rocksdb::FlushOptions options;
   options.ignore_if_flushed_after_tick = ignore_if_flushed_after_tick;
   bool flush_intents = intents_db_ && HasFlags(flags, FlushFlags::kIntents);