The rados operations now round-robin on the contexts to spread the load.
Modified the not yet used thread pool of BackendRados so it does not have its own context
anymore.

This will remove contention on the drive register as drives uipdate their statuses.
Also simplified structures in OStoreDB implementation (less references with added friend relations).

It was lowered during the watch based locking attempt.

This commit only replaces backoff with notifications in exclusive locks, for validation
with a multithreaded unit test. This proof of concept is considered as working, as the
measured gap between a lock release and a lock take never exceeds ~1/4 of a second, with a typical
release-lock gap in the few ms to few tens of ms.

This would detect instances where rados reports an object deletion before is actually happens.

The locks in Rados have timeouts. They are needed in case a locker process dies without
releasing its lock. As we have some contention in heavily loaded situations, it can happen
that a process is till accessing objects while the lock is expired. To lessen the likeliness
of this situation, the timeout has been increased from 10s to 60s.

The backoff was ajusted using the MultithreadLockingInterface unit test, with printouts
allowing to visually see the effect of the backoff strategy. The printouts are committed,
but they are commented out.

The same unit test was fized as it used to create an empty object, which is not supported
anymore in order to be able to detect locking of non-existing objects (lock creates the object,
but we detect non-existence as it is empty and re-delete it.

This mechanism of empty object locking detection is also added to the async update of object
as it was missing there (and the backoff has been added there too).

Added many conditional printouts in unit tests. Those printouts are controlled by #defines in a central file.

BackendVFS now throws the same exceptions as BackendRados.
The exceptions of the user provided update callback are now passed through.
A unit test was added.

Added an intermediate interface to reuse SchedulerDatabaseFactory.
Added optional debug infos for BackendVFS constructor.
The unit test is currently disabled.

The VFS implementation only launches the operation in the background (via std::async).
The Rados implementation uses the aio features of Rados. Lock and update are launched in the background via std::async.
Also now uses override for Backend virtual function.

This behavior is relied on is several places.

Disabled the tests for objectstore middle tier which will not be updated for a while, pending the middle tier interface review.
Remove the now useless (following the move to generic test) printouts of the type in backend store tests.
Added tests for RootEntry's admin hosts and admin users interface.

Created a richer protocol buffer RootEntry object and created a parital implementation to allow unit tests to run.

middletier, itself separated in interface, SQLite, objectstore and
shared tests.
Moved all utilities (exceptions, threading...) to a shared utility
directory.
Created a single, shared unit test from all the scattered ones.

License comment is no longer filtered out by doxygen when generating web pages of the individual source files.

Created AdminUsersList in the object store as a first step to implement an object store middle tier.
Some namespace fixes for unit tests.

and created initial unit tests for those.

Implemented the new interface on the Rados backend.
Fixed several bugs.