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.
Previously, chunk replicas were retreived with an SPI query. Now, all
catalog items are retrieved with native scans, with the exception of
newly created chunks.
This commit also refactors the chunk (replica) cache, removing some
data structures that were duplicating information. Now chunks are
cached by their ID (including their replicas) instead of just the
set of replicas. This removes the need for additional data structures,
such as the replica set which looked like a chunk minus time info,
and the cache entry wrapper struct. Another upside is that chunks
can be retrieved from the cache directly by ID.
