Switch autodiscover implementation from POX to SOAP

[ecederstrand]

Since version 2010, Exchange has provided an autodiscover implementation that uses SOAP. It offers some advantages, notably OAuth support which the POX implementation lacks, and allows us to reuse the code we already have to handle SOAP calls.

I think it's reasonable to drop support for autodiscovery on Exchange 2007 now, instead of maintaining two implementations.

Docs: https://docs.microsoft.com/en-us/exchange/client-developer/web-service-reference/soap-autodiscover-web-service-reference-for-exchange