Fix ConcurrentModificationException when user supplied Executor has more than 1 thread

[jnj]

The stale connection management assumes that its internal map will be accessed by only one thread at a time. But this is not always true; a user can provide an Executor that is backed by a thread pool. This synchronizes around the map put/remove to prevent ConcurrentModificationExceptions.

[pk11]

since webbit is single-threaded by design I believe it's assumed that the user supplied executor provides only a single thread as well. Please see https://groups.google.com/d/msg/webbit/Vr_MN9CwNsA/oaiidfkNv6wJ and https://github.com/webbit/webbit/blob/master/src/main/java/org/webbitserver/WebServers.java#L22 for more information.

[aslakhellesoy]

@pk11 that's correct. Webbit is not designed to work correctly when used with a multi-threaded executor, so this change is not necessary.