FTS message handler process is special system process, which doesn't
have access to catalog. Hence, it shouldn't be performing any catalog
access. Calling `superuser()` via `set_gp_replication_config()` or
`pg_reload_conf()` results in error for the same reason. Hence, if
`am_ftshandler` process avoid calling `superuser()` and thereby avoid
catalog access. Rest all the stuff performed by this process doesn't
need catalog access.

Fixes #4764 github issue.