Previously when using BETWEEN ... AND additional constraints in a
WHERE clause, the BETWEEN was not handled correctly because it was
wrapped in a BoolExpr node, which prevented plantime exclusion.
The flattening of such expressions happens in `eval_const_expressions`
which gets called after our constify_now code.
This commit fixes the handling of this case to allow chunk exclusion to
take place at planning time.
Also, makes sure we use our mock timestamp in all places in tests.
Previously we were using a mix of current_timestamp_mock and now(),
which was returning unexpected/incorrect results.
The extension state is not easily accessible in release builds, which
makes debugging issue with the loader very difficult. This commit
introduces a new schema `_timescaledb_debug` and makes the function
`ts_extension_get_state` available also in release builds as
`_timescaledb_debug.extension_state`.
See #1682
