Refactor sharedfileset.c to separate out fileset implementation.

Move fileset related implementation out of sharedfileset.c to allow its
usage by backends that don't want to share filesets among different
processes. After this split, fileset infrastructure is used by both
sharedfileset.c and worker.c for the named temporary files that survive
across transactions.

Author: Dilip Kumar, based on suggestion by Andres Freund
Reviewed-by: Hou Zhijie, Masahiko Sawada, Amit Kapila
Discussion: https://postgr.es/m/E1mCC6U-0004Ik-Fs@gemulon.postgresql.org

src/backend/replication/logical/launcher.c

@@ -648,6 +648,9 @@ logicalrep_worker_onexit(int code, Datum arg)
 
 	logicalrep_worker_detach();
 
+	/* Cleanup filesets used for streaming transactions. */
+	logicalrep_worker_cleanupfileset();
+
 	ApplyLauncherWakeup();
 }