Modified CMakeLists.txt files to apply -Wno-unused-function for GCC 6 and above.  It was GCC 7 and above before.

On a failed Send(), the client tries to reestablish the connection to the Server.

Done.

Done.

This setup allows reproducing a but seen in system tests.

This is validated in a unit test.

We now fall back to basic error codes when not supporting a specific extended error code.

The catalogue can receive polymorphic items as input for recording.
The strict sequencing of fSeqs is still enforced.
Currently, 2 items types are recorded: files and placehoders.
Files are recorded to the tape catalogue, while place holders are not,
but they are used to enforce the fSeq sequencing.

When no data is received for a given file, the tape server will
leave a placeholder on tape an carry on with the writing of the
following files, avoiding a tape unmount.

Updated unit test.

Fixed expected number of retries.

This is a stop gap solution for FTS tests which generate too many mounts as many file transfers
fail on checksum check and the file gets delete after the archive requests get generated.

On the longer run, we will need to imlement a placeholder mechanism where the taper server
can skip missing files and carry on with the migration session without unmounting the tape.

The current logging of failures for retrieve requests was attempting to retrieve the
list after the object was removed. The failures list has been moved to another log line,
similar to how it is done with archive requests.

Created initial (incomplete) logging of timings for generic popping from container.

Fixed missing variable initialization.
Added queue trimming.
Moved function from header to cpp.

The unit test is disabled as it does not pass yet.
Templated some structures to reflect imperfect similarities between inserting and popping.

Done.

The WrongPreviousOwner exception has been moved out from archive and retrieve requests to have more generic code.

Exits from Init() with exit(1) in case of failure instead of returning
false, as a temporary workaround until the following XRootD bug is
fixed:
https://github.com/xrootd/xrootd/issues/751

Done