When dropping the database schema, tables and sequences are only
dropped if they are present.

This commit avoids the message "table or view does not exist" by
modifying cta-catalogue-schema-drop to simply not try to delete
tables that do not exist.

cta/CTA#87 cta-catalogue-delete-all-data and cta-catalogue-schema-drop should require interactive confirmation

Removed the now redundant locking commands for the catalogue schema.
These commands were made redundant when the cta-catalogue-schema-drop
command was made interactive.

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.

cta/CTA#87 cta-catalogue-delete-all-data and cta-catalogue-schema-drop should require interactive confirmation

Made the cta-catalogue-schema-drop command interactive

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.