Now we are also tolerant of whether paths in config file do or do
not have a trailing slash.

Pathnames from the DB must have a leading slash if we have to strip off
the CASTOR prefix, but in any case we will convert all filenames to
absolute filenames by adding the EOS prefix.

EOS does not follow POSIX semantics for these bits and we are not even
sure if CASTOR honours them, so they will not be migrated.

Checksums should be sent as byte arrays not the string value

- CASTOR migration tools stay under CTA/migration
- upgrade tools are now under CTA/upgrade_db
- adds FindGRPC.cmake (required for migration tools)
- updates submodule to v0.50 (required for EOS gRPC API)