add the first implementation of an irods connection pool

it lives in the irodsstorage module and gets initialize at import.
it features one connection pool (as queue) per user,
who is identified by username and password.

It is not ready for anything serious yet, because:
1. the auth hash is not a real hash, so the pw is saved in clear
2. the dictionary accesses are not thread(multiuser)-safe
add crypto hash to store irods connection identifiers
rename authhash -> auth_hash
add a lock for manipulating the connection pool

lock the process of getting a user pool.
the user pool itself is threadsafe (as the Queue is threadsafe)
catch another peculiar irods exception

it's the cannot_delete_nonempty_homedir exception :P
print path of unittest objects rather than url
clean up unittests

remove calling generic get_url_list().
test will now fail when no valid backend is specified