change tracing to new transaction API in Gyrex

Signed-off-by: Gunnar Wagenknecht <gunnar@ageto.net>
Allow to cache connections for the current thread.

This commit introduced a thread-local connection cached which is can be
activated/deactivated as necessary. If active, the pool data source with 
cache and return a single connection as long as its active.

Signed-off-by: Gunnar Wagenknecht <gunnar@ageto.net>