refactor: Split "communicator" routines to a separate source file (#11459)

pagestore_smgr.c had grown pretty large. Split into two parts, such
that the smgr routines that PostgreSQL code calls stays in
pagestore_smgr.c, and all the prefetching logic and other lower-level
routines related to communicating with the pageserver are moved to a
new source file, "communicator.c".

There are plans to replace communicator parts with a new
implementation. See #10799.
This commit doesn't implement any of the new things yet, but it is
good preparation for it. I'm imagining that the new implementation
will approximately replace the current "communicator.c" code, exposing
roughly the same functions to pagestore_smgr.c.

This commit doesn't change any functionality or behavior, or make any
other changes to the existing code: It just moves existing code
around.

Author: hlinnaka

pgxn/neon/Makefile

@@ -4,6 +4,7 @@
 MODULE_big = neon
 OBJS = \
 	$(WIN32RES) \
+	communicator.o \
 	extension_server.o \
 	file_cache.o \
 	hll.o \