[Reviewer: Alex] Fix memcached ordering and improve SAS traces

The first part of this (i.e. the first commit) fixes https://github.com/Metaswitch/sprout/issues/662 using the approach agreed in this issue (overwriting av//impi/nonce to indicate that a REGISTER flow has finished successfully, not deleting it, which introduces a race condition).

The second part fixes https://github.com/Metaswitch/sprout/issues/664 - ensuring that AUTHENTICATION_TIMEOUTs get logged to SAS - and sorts out https://github.com/Metaswitch/sprout/issues/343 along the way. This means that:

* Call-ID markers are no longer raised for REGISTERs, SUBSCRIBEs and NOTIFYs
* The REGISTER-401, REGISTER-200 and any timeout flows are correlated together in SAS
* Subsequent unchallenged re-REGISTERs and de-REGISTERs appear in their own SAS trail
* NOTIFYs are logged in the same SAS trail as the message that triggered them (either a SUBSCRIBE or a REGISTER)

Testing:
* A call that looks the same as before: http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgmDhViQEEAAl_AUo=?sg_inst_id=7215
* Successful REGISTER: http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgaJuqxAACAAknAT8=?sg_inst_id=7224
* Failed REGISTER with timeout:http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgKJyuxAACAAiNASw=?sg_inst_id=7333
* Two re-REGISTERs close together, with the same Call-ID, but not correlated: http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgODJViQEEAAsXAX0=?sg_inst_id=7271, http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgODZViQEEAAsnAX8=?sg_inst_id=7272
* SUBSCRIBE triggering a NOTIFY: http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBgcJqsxAACAAb5APk=?sg_inst_id=7071
* REGISTER triggering a NOTIFY: http://sas.cw-ngv.com/serviceassurance/tview/view/eJxjZGBg8J6sxAACAAdBAQI=?sg_inst_id=7072

I found all these by just searching on the DN, so they're easily findable in the SAS UI.
Do you if sprout's route header has been added by this point? If not we will have problems correlating with Perimeta.