Adjust DSM and DSA slot usage constants (back-patch).

1.  Previously, a DSA area would create up to four segments at each size
before doubling the size.  After this commit, it will create only two at
each size, so it ramps up faster and therefore needs fewer slots.

2.  Previously, the total limit on DSM slots allowed for 2 per connection.
Switch to 5 per connection.

This back-patches commit d061ea21 from release 13 into 10-12 based on a
field complaint.

Discussion: https://postgr.es/m/CAO03teA%2BjE1qt5iWDWzHqaufqBsF6EoOgZphnazps_tr_jDPZA%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGL6H2BpGbiF7Lj6QiTjTGyTLW_vLR%3DSn2tEBeTcYXiMKw%40mail.gmail.com

src/backend/storage/ipc/dsm.c

@@ -45,13 +45,8 @@
 
 #define PG_DYNSHMEM_CONTROL_MAGIC		0x9a503d32
 
-/*
- * There's no point in getting too cheap here, because the minimum allocation
- * is one OS page, which is probably at least 4KB and could easily be as high
- * as 64KB.  Each currently sizeof(dsm_control_item), currently 8 bytes.
- */
 #define PG_DYNSHMEM_FIXED_SLOTS			64
-#define PG_DYNSHMEM_SLOTS_PER_BACKEND	2
+#define PG_DYNSHMEM_SLOTS_PER_BACKEND	5
 
 #define INVALID_CONTROL_SLOT		((uint32) -1)
 

src/backend/utils/mmgr/dsa.c

@@ -74,7 +74,7 @@
  * dsm.c's limits on total number of segments), or limiting the total size
  * an area can manage when using small pointers.
  */
-#define DSA_NUM_SEGMENTS_AT_EACH_SIZE 4
+#define DSA_NUM_SEGMENTS_AT_EACH_SIZE 2
 
 /*
  * The number of bits used to represent the offset part of a dsa_pointer.