Drop log level for incorrect logging contexts to WARN if the context is wrong and DEBUG if the context is missing

This should fix the bug where the edges of the graph get returned in a
different order than they were inserted in, and so no get_event no
longer returned the exact same JSON as was inserted. This meant that
signature checks failed.

SYN-58: Allow passing explicit limit=0 to initialSync to request no messages at all; missing still implies default 10

Http client refactor

Separate out the matrix http client completely because just about all of its code it now separate from the simple case we need for standard HTTP(S)

Use module loggers rather than the root logger. Exceptions caused by bad clients shouldn't cause ERROR level logging. Fix sql logging to use 'repr' rather than 'str'

Add a few missing yields, Move deferred lists inside PreserveLoggingContext because they don't interact well with the logging contexts