Issue 4797:Prevent thread leak when BookKeeper client constructor fails

[kroosxu]

Descriptions of the changes in this PR:

Fix #4797

Motivation

When BookKeeper.forConfig(conf).build() fails part-way through construction
(for example because metadataServiceUri points to an unreachable or invalid
ZooKeeper ensemble, or MetadataDrivers#getClientDriver(URI) cannot resolve
a driver for the configured scheme), the constructor leaks every resource it
has already allocated up to the point of failure.

Changes

• In the BookKeeper constructor, when initialization fails after the
  worker pools / bookie client have been created but before the object is
  fully constructed, invoke close() on the partially-initialized instance
  so that all already-allocated resources are
  shut down before the exception is propagated to the caller.

• Add BookKeeperConstructorFailureTest under
  bookkeeper-server/src/test/java/org/apache/bookkeeper/client/ covering
  the regression:

  • testNoThreadLeakWhenMetadataServiceUnavailable — a single failing
    build() must not leave any client-owned thread behind.
  • testRepeatedFailedBuildsDoNotAccumulateThreads — 20 consecutive
    failing build() calls must not accumulate threads.