Make Releasable extend AutoCloseable.

Java7's AutoCloseable allows to manage resources more nicely using
try-with-resources statements. Since the semantics of our Releasable interface
are very close to a Closeable, let's switch to it.

Close #5689
Ensure close is called under lock in the case of an engine failure

Until today we did close the engine without aqcuireing the write lock
since most calls were still holding a read lock. This commit removes
the code that holds on to the readlock when failing the engine which
means we can simply call #close()