The apscheduler background scheduler spawns a native thread
which is not monkey patched which interacts with shared module
level objects like the module level LOG instances and sqlachmey
engine facades.
This is unsafe and leads to mixing patched and unpatched
code in the same thread.
This manifests in 2 ways:
1.) https://paste.opendev.org/show/bGPgfURx1cZYOsgmtDyw/
sqlalchmey calls can fail due to a time.sleep(0) in oslo.db being invoked
using the unpatched time modules in an eventlet greenthrad.
2.) https://paste.opendev.org/show/b5C2Zz4A4BFIGbKLKrQU/
over time that caused the sqlalchmy connection queuepool to fill up preventing
backgound tasks form running like reconsiling audits.
This change adresses this by overloading the background scheduler _main_loop
to monkey patch the main loop if the calling thread was monkey patched.
Closes-Bug: #2086710
Change-Id: I672c183274b0a17cb40d7b5ab8c313197760b5a0
f07694ba6c