add more debugging messages to irods storage
fix the irods connection reuse; more debug messages

irods connections were not reused, because we forced
creating a new connection whenever the used connections
are 0. It should be only when the available connections,
the ones in the applicable user pool, are 0.

add more debug messages and display the current sizes
of a user pool and of the usedconnections set in the
debug messages.
refactor the use of irods connections

this patch changes:
1. the use of multiple connections in one request is now safe
2. irods connection do not make use of the g object,
  the request stack
3. we do not need a teardown_request function anymore

This changes the way irods connections are handled.
The authenticate function no longer puts a connection on the g stack,
but stores it in the connection pool. The rest of the API functions
take it from there, and can also initiate the creation of more
connections, if necessary.