Some important bug fixes

Manual tester: No string resizing after recvfrom.
Broker: Forgot to send persistence command on LOGOUT.
Variadic debug_log; picked manual-tester fix from master
Some more minor debug_log improvements
Dirty thread support (no locks, just parallel)

Works fine so far: different threads receive/send messages (not only
one), errors are caught and logged.
Requirements updated: Some more boost libraries.
Synchronization code (infrastructure)
Introduced locking for files (urandom, stderr)
The actual broker uses locks now.

We use a mix of exclusive and shared locks: Read access needs only a
shared lock, so many threads may read concurrently. However, write
accesses are protected by exclusive locks; before a thread can write to
a map, it has to wait for a) all readers or b) another writer before it
can continue.
Merge branch 'mesg-broker-threaded' into mesg-broker