Heartbeat should not reset, when pool is full

[rPraml]

We discovered an issue, when the connection pool is full, the heartbeat will reset the pool, although this is not necessary.

How to reproduce:

• create a pool (e.g. maxSize = 1)
• start as many threads as maxSize that will hold the connection longer than heartbeatfreq + waitTimeoutMillis
• heartbeat will not get a connection and thinks, datasource is down, which is not true.

This PR changes, how heartbeat detects, if connection is valid.

• If the pool is full, a ConnectionPoolExhaustedException extends SQLException is thrown
• If the hearbeat tries to validate the connection and gets a ConnectionPoolExhaustedException it will be ignored multiple times.

We ignore this exception heartbeatMaxPoolExhaustedCount times. Normally this is not needed, but if there are really leaking connections (e.g. forgot to close) we perform a reset, if this happened repeatetly