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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f5332503ef0a65b90366c2a222e1b5aa64977a95

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(-)