Reimplement free-space-map management as per recent discussions.

Adjustable threshold is gone in favor of keeping track of total requested
page storage and doling out proportional fractions to each relation
(with a minimum amount per relation, and some quantization of the results
to avoid thrashing with small changes in page counts).  Provide special-
case code for indexes so as not to waste space storing useless page
free space counts.  Restructure internal data storage to be a flat array
instead of list-of-chunks; this may cost a little more work in data
copying when reorganizing, but allows binary search to be used during
lookup_fsm_page_entry().

src/backend/utils/misc/postgresql.conf.sample

@@ -48,10 +48,11 @@
 #	Shared Memory Size
 #
 #shared_buffers = 64		# min max_connections*2 or 16, 8KB each
-#max_fsm_relations = 1000	# min 10, fsm is free space map, ~40 bytes
-#max_fsm_pages = 10000		# min 1000, fsm is free space map, ~6 bytes
 #max_locks_per_transaction = 64	# min 10
 #wal_buffers = 8		# min 4, typically 8KB each
+# fsm = free space map
+#max_fsm_relations = 1000	# min 100, ~50 bytes each
+#max_fsm_pages = 20000		# min max_fsm_relations*16, 6 bytes each
 
 #
 #	Non-shared Memory Sizes