#131 Expiring connections may cause unnecessary acquire

[Robert Wruck]

Assume a connection pool with an initial and minimum size of 0.
When trying to check out the first connection, the target_pool_size gets incremented to 1 to acquire a new connection. If acquiring this connection fails, the target_pool_size is not decremented again.
Since the expiry check (cullExpired) will try to ensure the connection pool's minimum size, each expiry check from now on will cause a new unnecessary acquire attempt though no clients are waiting for a checkout.

(using c3p0-0.9.2.1, but 0.9.5 contains no relevant change)