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

Preserve logging context in a few more places, drop the logging context after it has been stashed to reduce potential for confusion

for all pdus sent.