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

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2ded1f1fbb6e8e2b7927cce93238775604069bc3

Modified Files
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src/backend/storage/ipc/dsm.c | 7 +------
src/backend/utils/mmgr/dsa.c  | 2 +-
2 files changed, 2 insertions(+), 7 deletions(-)