Azure objstorage performance improvements
Most notably:
Add a prefixed azure object storage which directly maps across several accounts
This should reduce the overhead from using the stack of multiplexer and filter to access our Azure content stores.
Before adding this, stack a few cleanup commits, most notably:
- Use a request_session for the block blob service to enable https pipelining
- objstorage_azure: Cleaner mocking of the BlockBlobService class for tests
- Use get_objstorage instead of instantiating the AzureCloudObjStorage class directly in test
This is related to #1044 (closed)
Test Plan
Added unit tests
Migrated from D318 (view on Phabricator)