[BREAKING] Refactor NimBLEServer

* General code cleanup
* `NimBLEServerCallbacks::onMTUChanged` renamed to `NimBLEServerCallbacks::onMTUChange` to be consistent with the client callback.
* `NimBLEServer::getPeerIDInfo` renamed to `NimBLEServer::getPeerInfoByHandle` to better describe it's use.
* Use a std::array to store client connection handles instead of std::vector to reduce memory allocation.
* `NimBLEServer::disconnect` now returns `bool`, true = success,  instead of `int` to be consistent with the rest of the library.

Author: h2zero