Changed 'catch (exception::Exception ex)' to 'catch (exception::Exception &ex)' in order to avoid error: catching polymorphic type ‘class cta::exception::Exception’ by value

(see 'ps -ax -o "pid comm cmd"  | grep /usr/bin/cta-taped').

Sub request support to be added.

The maintenance process replaces the garbage collector process. It now
runs both garbage collection and disk reports.

Each thread has its own rados context. The thead pool, when used will allow better
parallelism of asynchronous requests.

This required some rework on the backend creations as the threads will need logging.

The previous strategy was made under the assumption that we needed to lock sparingly.
With the introduction of lockfree strategy, this is not true anymore.
The new strategy will be immune from the A watches B, B watches A, both die, and
no one garbage collects them situation (also called cyclers in utility cta-objectstore-unfollow-agent).

For cta-taped.conf and cta-frontend.conf use the same parameter
ObjectStore BackendPath .......

The ping() and queueRetrieve() timings are of interest, so commit now.
The rest of the timings will follow.

In order to be efficient, the data (tape status and queue sizes) is cached in memory.
This function needed access to the catalogue (where the tape status is located).
This in turn required passing the catalogue object to:
 - The garbage colletion functions for all object types.
 - The garbage collector object now keeps a reference to the catalogue.
 - The path to the catalogue credential file should now be passed to cta-objectstore-collect-orphaned-object
 - The OStoreDB also needs a reference to the catalogue in order to queue retrive requests.

 The garbage collection member function are now also being passed a log context, so the requirement for better
 log of object fate can also be fulfilled later. This in turn required the passing of a logger object to
 the object store and relative helper classes.

 Finally, a dummy catalogue class has been created to be used in unit tests.

Implemented GarbageCollector::reinjectOwnedObject() and called it on garbage collector shutdown.
This fixes the issue of non-empty garbage collectors exiting.

This required to pass an AgentReference through to all garbageCollect() member functions of all objects.
The changed behavior required adaptation of the corresponding unit test (the archive queues now need to
be properly referenced in the root entry).

The different processes of taped can now be identified as in this exemple:
[eric@localhost ~]$ ps -ax -o "pid comm cmd"  | grep /usr/bin/cta-taped
[...]
32593 cta-taped-main  /usr/bin/cta-taped -fl /cta-taped.log
32800 cta-tpd-VDSTK11 /usr/bin/cta-taped -fl /cta-taped.log
32801 cta-taped-gc    /usr/bin/cta-taped -fl /cta-taped.log

Implemented Agent::garbageCollect()
Made AgentWatchdog fetch the timout from the agent object.
Fixed unconditional time tounter reset in AgentWatchdog::checkAlive()
Fixed missing log for garbageCollection.
Added utility (cta-objectstore-unfollow-agent) to mark agents as not followed when they get orphaned. This utility detects looping ownerships (two garbage coolectors watch each other and both die synchronously). This utility is not packaged.
Various log improvements.

Added missing garbage collection for all object types.
Added logging to AgentHeartbeat.
Some object types still miss garbage collection implementation.

Added logging in case of exception in GC.
Fixed wrong GC handler behaviour when subprocess exits/crashes.

Also removed the heartbeat from the GC's main functions (it is now handled in a separate thread).

Added missing destructor for GCHandler.
Added subprocess to taped's process manager.

Created a garbage collector subprocess for the drive daemon.
Created a AgentHeartbeat thread and added it to frontend, drive subprocess and GC subprocess.
Fixed logs and drive subprocess.
Renamed common/threading/Threading.[hc]pp to Thread.[hc]pp as there is only one class left in this file.