This PR disables query optimizations on regular tables by default.
The option timescaledb.optimize_plain_tables = 'on' enables them
again. timescaledb.disable_optimizations = 'on' disables all
optimizations (note the change from 'true' to 'on').
Common time-series rollups are of the form SELECT date_trunc(const,
time) as t, aggregates... GROUP BY t ORDER BY t DESC LIMIT const2.
Previously, Postgres would not optimize such queries because it
could not match date_trunc(const, time) to any indexes on time, since
it has no way to know that a sort on time is always a valid sort on
date_trunc(const, time).
This commit implements this optimization for date_trunc but it could
apply to a wider range of functions, to be implemented later.
