REFACTOR: add is_in_transaction() function in cvmfs_server
REFACTOR: introduce open_transaction() and close_transaction()
FIX: missing parameter for is_stratum0
FIX: typo
shield against interleaving publishes
TEST: interleaving publish operations
Merge pull request #384 from reneme/fix-interleaving-publish

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.

Jira Ticket: https://sft.its.cern.ch/jira/browse/CVM-264