pgsql: Recalculate search_path after ALTER ROLE.

Recalculate search_path after ALTER ROLE.

Renaming a role can affect the meaning of the special string $user, so
must cause search_path to be recalculated.

Discussion: https://postgr.es/m/186761d32c0255debbdf50b6310b581b9c973e6c.camel@j-davis.com
Reviewed-by: Nathan Bossart, Michael Paquier
Backpatch-through: 11

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fa2e874946c5b9f23394358c131e987df7cc8ffb

Modified Files
--------------
src/backend/catalog/namespace.c                   |  6 +-
src/test/isolation/expected/search-path-inval.out | 97 +++++++++++++++++++++++
src/test/isolation/isolation_schedule             |  1 +
src/test/isolation/specs/search-path-inval.spec   | 59 ++++++++++++++
4 files changed, 162 insertions(+), 1 deletion(-)