FIX: cvmfs_server is Vulnerable to Interleaving Publish Operations

This adds a lock mechanism around the publishing (respectively rollbacking) in `cvmfs_server`. Additionally it replaces the formerly used sentinal file `in_transaction` by an analogly named directory. This is done, since `mkdir` checks the existance and creates the directory as an atomic operation.

This comes with a test that tries to invoke an interleaving publish operation.