EXPLAIN; pg-sql2 enhancements; pgSelect/pgUnionAll query overhaul; JIT tweaks

[benjie]

• Add EXPLAIN support, including EXPLAIN ANALYZE (TODO: make this configurable from Ruru!)
• Fix accidental SQL trimming in Ruru
• pg-sql2 now uses the same placeholder for the same sql.value node (i.e. $1 ... $1 rather than $1 ... $2)
• Completely refactor how pgSelect/pgUnionAll builds the SQL queries, and how PgExecutor executes them
  • no more queryValuesSymbol hack - instead we just push to an array
  • now we have two queries: one for when there's only one "pending" (faster) and one where there's more than one
• Replace set jit = 'off' with set jit_optimize_above_cost = -1
  • it turns out that the JIT can significantly help in the case of unindexed queries, with it turned off some queries were 20+% slower than in V4 purely due to JIT differences.
• pg-sql2 now makes symbolToIdentifier available to consumers