Rename gec13 to poster-gec13
Added a vacuumer to the UG.
* When a UG writes data, the MS creates a log of the manifest timestamp and the affected blocks at the time of the write.
* When a UG garbage-collects data, it removes the associated log record from the MS.
* If a UG fails to garbage-collect data (e.g. it crashes or becomes unavailable), it will poll the head of each file's write log from the MS, and garbage-collect the blocks and manifests it had previously failed to do.
* To ensure that no memory leaks occur, a file cannot be unlinked or renamed over if it has a non-empty write log.  The coordinator UG must read the log and garbage-collect (i.e. "vacuum") all data prior to removing the entry from the namespace.

Other bugfixes:
* RG disk driver: return 404 instead of 500 if a block or manifest doesn't exist.
* RG: remove deletion receipts
* update TODO.txt with notes for future deployment scenarios
* UG: refactor manifest download logic
* UG: fix memory leak of sync_ctx on close
* UG: add a "deletion in progress" flag to fs_entry, so we don't block the whole world while vacuuming a file
* UG: added support for continuations in replication.cpp.  Client modules (like vacuumer.cpp and sync.cpp) can register callbacks to be invoked once a replica_context has been processed.
* UG: added API to differentiate between asynchronous, backgrounded, and synchronous options for replication and garbage collection
* UG: consolidate WriteMsg types.  Do away with ACCEPT and PREPARE in favor of just WRITE.
* libsyndicate: refactor MS communication methods, consolidating duplicated logic into ms_client_read() and ms_client_write() to handle low-level HTTP operations.
* MS: change "== None" to "is None", and "!= None" to "is not None"
* MS: replace GCPEEK with VACUUM handler
* MS: remove deletion receipt logic