Avoid query-lifetime memory leaks in XMLTABLE (bug #15321)

Avoid query-lifetime memory leaks in XMLTABLE (bug #15321)

Multiple calls to XMLTABLE in a query (e.g. laterally applying it to a
table with an xml column, an important use-case) were leaking large
amounts of memory into the per-query context, blowing up memory usage.

Repair by reorganizing memory context usage in nodeTableFuncscan; use
the usual per-tuple context for row-by-row evaluations instead of
perValueCxt, and use the explicitly created context -- renamed from
perValueCxt to perTableCxt -- for arguments and state for each
individual table-generation operation.

Backpatch to PG10 where this code was introduced.

Original report by IRC user begriffs; analysis and patch by me.
Reviewed by Tom Lane and Pavel Stehule.

Discussion: https://postgr.es/m/153394403528.10284.7530399040974170549@wrigleys.postgresql.org