Only offer msisdn flows if the x_show_msisdn option is given.

Changes from https://github.com/matrix-org/synapse/pull/1971

Add helpful texts to logger config options

Refactor FederationServer._handle_new_pdu

Add an option to disable stdio redirect

Reread log config on SIGHUP

This makes it tractable to run synapse under pdb.

- to make it easier to add more config options.

When we are using a log_config file, reread it on SIGHUP.

Noop repated delete device inbox calls from /sync

Unfortunately this significantly increases the size of the already-rather-big
FederationHandler, but the code fits more naturally here, and it paves the way
for the tighter integration that I need between handling incoming PDUs and
doing the join dance.

Other than renaming the existing `FederationHandler.on_receive_pdu` to
`_process_received_pdu` to make way for it, this just consists of the move, and
replacing `self.handler` with `self` and `self` with `self.replication_layer`.

When we receive PDUs via `get_missing_events`, we have already checked their
sigs, so there is no need to do it again.

This should be functionally identical: it just seeks to improve readability by
reducing indentation.

Support registration & login with phone number

This file post-dates OM

looks like the upsert function was accidentally sending sql logging to the
general logger. We already log the sql in `txn.execute`.

and replace requestEmailToken where we meant requestMsisdnToken

Plus a couple of other minor fixes

https://docs.google.com/document/d/1-6ZSSW5YvCGhVFDyD2QExAUAdpCWjccvJT5xiyTTG2Y/edit#