Protect Blocking API

Currently, the UaaSigningKeyResolver must use .block() in order to implement a
blocking interface.  Use of this API within a non-blocking thread is illegal
since Reactor 3.2.  A .scheduleOn() call, moving the execution to an elastic
scheduler was in place but turned out to be the wrong implementation.  This
change updates the protection to a .publishOn() to move it out of a
non-blocking thread at the right time.

Signed-off-by: Ben Hale <bhale@vmware.com>

Author: nebhale

cloudfoundry-client-reactor/src/main/java/org/cloudfoundry/reactor/uaa/UsernameProvider.java

@@ -49,8 +49,8 @@ final class UsernameProvider {
 
     Mono<String> get() {
         return getToken(this.connectionContext, this.tokenProvider)
+            .publishOn(Schedulers.elastic())
             .map(this::getUsername)
-            .subscribeOn(Schedulers.elastic())
             .retry(1, t -> {
                 if (t instanceof ExpiredJwtException) {
                     this.tokenProvider.invalidate(this.connectionContext);