retain NextException

[nitincchauhan]

two changes:

1. in getLastConnectionFailure() it should be just get() and not getAndSet(null)
2. nullify it after successful isValid()

see http://stackoverflow.com/questions/39479345/how-do-i-know-if-my-hikaricp-pool-is-really-exhausted-when-sqltransientconnectio

[brettwooldridge]

@nitincchauhan Agreed on getAndSet(null), but resetting in isValid() is not correct. The current reset in newConnection() is sufficient.

[nitincchauhan]

please nullify after successful isConnectionAlive too, just like newConnection(). otherwise getConnection() may report 'stale' exception and it can not be used to distinguish as pool exhausted or db problem

once set in isConnectionAlive() for one bad connection, even if pool is exhausted, it would carry same exception till pool adds new connection.

Also, probably setting nextException is redundant and polluting logs because 'cause is set as nextException' anyway. printing exception also prints cause, as ...caused by...