Add @maintenance decorator to methods that cannot be executed concurrently

[giovannipizzi]

Add a guard to avoid maintenance methods (repack, delete, add directly to packs) are performed in parallel.

Add decorator that opens a lock file in exclusive mode, and stops if someone else is already performing maintenance operations.

Possibly check (both this, and the one that locks a pack) that they have a finally block that releases the lock.

[giovannipizzi]

Also:

• check if there is a second level of locking, e.g. when deleting?
• until Problem during concurrent tests #4 is fixed, check if we need to prevent also normal usage of the repository by any process while in maintenance? (E.g. with a lock file, containing the PID, and in case of failure of the packing process, we explain how to check if it's still running, and how to delete the lock).

[giovannipizzi]

In AiiDA, we will try to put all safeguards to avoid problems. Putting a decorator properly is a bit tricky, so I am moving this to a 'nice to have'. This will need to be properly documented, though...

[giovannipizzi]

After careful thinking: while one could implement guards (e.g. a decorator @maintenance for the relevant methods) to prevent concurrent access (see e.g. issue #6), this is complex to
implement (mostly because one needs to record any time a process starts accessing the repository - so a maintenance
operation can refuse to start - as well as check if any maintenance operation is running at every first access to a
container and refuse to start using it).
While not impossible, this is not easy (also to properly clean up if the computer reboots unexpectedly when a
maintenance operation is running, etc) and also might have performance implications as a check has to be performed
for every new operation on a Container.

We note that however such logic was implemented in AiiDA
(that uses disk-objectstore as a backend). Therefore guards are in place there, and if one needs to do the same
from a different code, inspiration can be taken from the implementation in AiiDA.

I will therefore close this, and the docs will be added as part of #3