Lock table in ShareUpdateExclusive when importing index stats.

Lock table in ShareUpdateExclusive when importing index stats.

Follow locking behavior of ANALYZE when importing statistics. In
particular, when importing index statistics, the table must be locked
in ShareUpdateExclusive mode. Fixes bug reportd by Jian He.

ANALYZE doesn't update statistics on partitioned indexes, and the
locking requirements are slightly different for in-place updates on
partitioned indexes versus normal indexes. To be conservative, lock
both the partitioned table and the partitioned index in
ShareUpdateExclusive mode when importing stats for a partitioned
index.

Author: Corey Huinker
Reported-by: Jian He
Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CACJufxGreTY7qsCV8%2BBkuv0p5SXGTScgh%3DD%2BDq6%3D%2B_%3DXTp7FWg%40mail.gmail.com