pgsql: Fix leak with SMgrRelations in startup process

Fix leak with SMgrRelations in startup process

The startup process does not process shared invalidation messages, only
sending them, and never calls AtEOXact_SMgr() which clean up any
unpinned SMgrRelations.  Hence, it is never able to free SMgrRelations
on a periodic basis, bloating its hashtable over time.

Like the checkpointer and the bgwriter, this commit takes a conservative
approach by freeing periodically SMgrRelations when replaying a
checkpoint record, either online or shutdown, so as the startup process
has a way to perform a periodic cleanup.

Issue caused by 21d9c3ee4ef7, so backpatch down to v17.

Author: Jingtang Zhang <mrdrivingduck@gmail.com>
Reviewed-by: Yuhang Qiu <iamqyh@gmail.com>
Discussion: https://postgr.es/m/28C687D4-F335-417E-B06C-6612A0BD5A10@gmail.com
Backpatch-through: 17

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/039301b3fceb00a8c7c6cdf8ccbfaf912dd8cd5f

Modified Files
--------------
src/backend/access/transam/xlog.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)