Removed castor-rmc-server rpm from buildtree-stage1-rpms/Dockerfile because CTA now builds /usr/bin/cta-rmcd

mediachanger/castorrmc/CMakeLists.txt
mediachanger/castorrmc/common/CMakeLists.txt
mediachanger/castorrmc/rmc/CMakeLists.txt

Please note that casting the result to void is not good enough for gcc >= 7

Added -Wno-narrowing -Wno-implicit-fallthrough compiler flags to protoc3 output files if using gcc version 7 or greater

We now assume that Finished() will always be called, so explicit timeout
handling in RequestProc is redundant.

In case of successful Request processing, the XRootD framework will call
Finished(cancel=false). In the case of a failure or timeout, the framework
will call Finished(cancel=true).

Fixed errors generated by Scheduler code passing getMessage() instead of getMessage().str() to ScopedParamContainer.add()

mediachanger code.

Example error:

/home/smurray/CTA/mediachanger/io.hpp:353:3: error: dynamic exception
specifications are deprecated in C++11 [-Werror=deprecated]
   throw(cta::exception::TimeOut, cta::exception::Exception);
   ^~~~~

This ensures the uniqueness of the agent name even if a process creates an agent several times in the same second.

Forwarding -b and -B options to `create_instance.sh` so that we can run the buildtree test directly like this:
```
./run_systemtest.sh  -n toto -s tests/archive_retrieve.sh -O -D -k -b /buildtree -B CTA-build
```
This will be very helpful to test buildtree in CI automatically.