Save time of last GC in XAPI

[olivierlambert]

Somehow related to #524

What do you think of storing the last timestamp of when a GC was triggered on a specific SR? (like a new field in XAPI or just a record in other_config).

This might be helpful to track discrepancies between a chain that have to be coalesce and last time it was done (and likely detects SR issues before it's too late!)