The framework now expects only 3 protobuf types (Request, Metadata-response
and Alert). This is because the response payload will be arbitrary data of
arbitrary length and so is less well suited for protocol buffers.

We can define the characteristics of the response (size, format, etc.) in
the Metadata protobuf, or (if there is a use case), we can split the response
into records and use a protobuf for the record format.

A top-level string which is unassigned will be assigned a default value of empty
string. If we put it inside a nested message, then an unassigned string has no
value.

No wrapper for sent messages (as only one message type can be sent). Defines a
wrapper for received messages. Defines message types for errors and alerts.

Changes to protobuf breaks cta-xrootd_plugins-write-notification-msg, so
I removed it from the build in this branch.

Implemented GarbageCollector::reinjectOwnedObject() and called it on garbage collector shutdown.
This fixes the issue of non-empty garbage collectors exiting.

Simplifies code and avoids unnecessary string copy construction

We now only do one queue update at a time (per process). While one batch is being pushed to the objet store, the
following one is being built up, and written as soon as the previous write completes. This can be see in the timings from
the logs of the unit test OStoreSchedulerDatabaseTestVFS/SchedulerDatabaseTest.createManyArchiveJobs/0 here:
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-68" sizeBefore="0" sizeAfter="1" addedJobs="1" waitTime="0.002403" enqueueTime="0.036594"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-68" sizeBefore="1" sizeAfter="100" addedJobs="99" waitTime="0.036547" enqueueTime="0.001629"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="0" sizeAfter="1" addedJobs="1" waitTime="0.000058" enqueueTime="0.061447"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="1" sizeAfter="198" addedJobs="197" waitTime="0.061434" enqueueTime="0.001431"
[...] MSG="In MemArchiveQueue::sharedAddToArchiveQueue(): added batch of jobs to the queue." objectQueue="archiveQueue-[...]-174" sizeBefore="198" sizeAfter="200" addedJobs="2" waitTime="0.000840" enqueueTime="0.037259"

twice and outputs EOS workflow proc content to list various type of
errors.

It appeared in production that the update latency could be steadily higher than this delay.
This led to an ever growing perceived latency by the client, as the batches to be queued accumulated
faster than they were drained.

This is an interim solution to a more adaptive one which should make no asumption.

Handling of multi-block responses and streaming is not yet implemented.

Renames private member variables. Changes to some constructor arguments.

First character of XrdSsi metadata is overwritten with zero, see
https://github.com/xrootd/xrootd/issues/537
This workaround adds a dummy character to the front of the metadata on
the server side and removes it on the client side.

Added computation of files on tapes (based on last Fseq).
Fixed computation of writable tapes count.

Now waits n=30 secs for any outstanding requests to finish before
giving up and executing a forced shutdown.

Wrong parameter passing fixed.

Added tape statistics for retrieves.
Added oldest jobs age.