DROP EXTENSION didn't properly reset caches and other saved state
causing various errors related to bad state when the extension was
dropped and/or recreated later.
This patch adds functionality to track the state of the extension and
also signals DROP EXTENSION to other backends that might be running,
allowing them to reset their internal extension state.
This patch refactors the source code so that a bunch of unrelated code
for the planner, process utilities and transaction management, which
was previously located in the common file timescaledb.c, is now broken
up into separate source files.
