Avoid an HBase connection mgmt error

HBase logged an error when we tried to close a connection which didn't exist anymore, because its refcount reached 0 earlier. Simply removing the places where we explicitly deleted connections solves this. For LilyClient, if any connections would leak we have HBaseConnections.forceClose in place (which won't print an error since it first retrieves/creates a connection).

Added a testcase which creates multiple LilyClients and verifies they don't close each other's connections.

Thought a while about doing a bigger refactoring, but concluded that what we have now isn't too bad.