Global snapshots

Check that global_snapshot_defer_time early instead of falling on assert.

Also set default value to 30 secs.

Mostly cosmetical improvements.

 * (Arguably) improved comments around locking during circular buffer
   maintenance; also, don't lock procarray during global_snapshot_xmin bump.
 * s/snaphot/snapshot, other typos.
 * Don't track_global_snapshots by default -- while handy for testing, it
   doesn't look generally good.

(cherry picked from commit f9dd1c3)

Fix initdb failure with non-zero global_snapshot_defer_time.

And set its default value to 5.

Also fix one warning (damned 'void' instead of true void).

(cherry picked from commit d545692)

Author: kelvich

src/backend/access/transam/Makefile

@@ -12,7 +12,7 @@ subdir = src/backend/access/transam
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = clog.o commit_ts.o global_csn_log.o generic_xlog.o \
+OBJS = clog.o commit_ts.o global_csn_log.o global_snapshot.o generic_xlog.o \
 	multixact.o parallel.o rmgr.o slru.o \
 	subtrans.o timeline.o transam.o twophase.o twophase_rmgr.o varsup.o \
 	xact.o xlog.o xlogarchive.o xlogfuncs.o \