xrootd.spec.in in the XRootD SSI branch does not package the SSI
libraries/headers. I'm putting a patched version here temporarily until
Michal or Andy can fix it in the main XRootD repository.

Done.

Following https://gitlab.cern.ch/cta/CTA/commit/f38da19dfe5f7312cf6e5b3b4a2028121d7b6832#note_210679
Also gets repos from docker.
bootstrap now requires a complete checkout.

The next drive process was hence unaware of a crash an missed the oportunity to clean the drive.

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.

This reverts commit 363c3214.

Also made garbage collection faster.

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.