* add a GET /accounts/{a}/boostrap URI that delivers the same as GET /
but also mailboxes for a given account, in case the UI remembers the
last used account identifier, to avoid an additional roundtrip
* streamline the use of simpleError()
* add logging of errors at the calling site
* add logging of evictions of Sessions from the cache
* change default Session cache TTL to 5min instead of 30sec
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