add close=False parameter to engine.dispose()

add close=False parameter to engine.dispose()

Added new parameter :paramref:`.Engine.dispose.close`, defaulting to True.
When False, the engine disposal does not touch the connections in the old
pool at all, simply dropping the pool and replacing it. This use case is so
that when the original pool is transferred from a parent process, the
parent process may continue to use those connections.

Fixes: #7877
Change-Id: I88b0808442381ba5e50674787cdb64f0e77d8b54