Functions marked IMMUTABLE should also be parallel safe, but
aren't by default. This change marks all immutable functions
as parallel safe and removes the IMMUTABLE definitions on
some functions that have been wrongly labeled as IMMUTABLE.
If functions that are IMMUTABLE does not have the PARALLEL SAFE
label, then some standard PostgreSQL regression tests will fail
(this is true for PostgreSQL >= 10).
Aggregate functions that have serialize and deserialize support
functions (histogram, last, first, etc.) should have these
support functions marked STRICT.
PostgreSQL's regular test suite will fail when the timescaledb
module is loaded without these functions being marked STRICT.
