Fix signal handling in logical replication workers

The logical replication worker processes now use the normal die()
handler for SIGTERM and CHECK_FOR_INTERRUPTS() instead of custom code.
One problem before was that the apply worker would not exit promptly
when a subscription was dropped, which could lead to deadlocks.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>

src/include/replication/logicalworker.h

@@ -14,4 +14,6 @@
 
 extern void ApplyWorkerMain(Datum main_arg);
 
+extern bool IsLogicalWorker(void);
+
 #endif   /* LOGICALWORKER_H */