Create context-specific Tsrcache.

Not used yet.
Modify SPI to use Tsrcache instead of List.
Pass the Tsrcache through ParserState to parse analysis.
Use executor state rather than SPI to get named tuplestores.

spi.c and trigger.c will need a little more clean-up to match this,
and it's likely that not all places that need to pass the baton are
doing so, but at least this passes regression tests.