* implement correct Etag and If-None-Match handling, responding with
304 Not Modified if they match
* introduce SessionState and State string type aliases to ensure we are
using the correct fields for those, respectively
* extract the SessionState from the JMAP response bodies in the
groupware framework instead of having to do that in every single
groupware API
* use uint instead of int in some places to clarify that the values are
>= 0
* trace-log how long a Session was held in cache before being evicted
* add Trace-Id header handling: add to response when specified in
request, and implement a custom request logger to include it as a
field
* implement a more compact trace-logging of all the methods and URIs
that are served, to put them into a single log entry instead of
creating one log entry for every URI
* ensure that all the jmap responses contain the SessionState
* implement missing errors that were marked as TODO
* moved common functions from pkg/jmap and pkg/services/groupware to
pkg/log and pkg/structs to commonalize them across both source trees
* implement error handling for SetError occurences
* Email: replace anonymous map[string]bool for mailbox rights with a
MailboxRights struct, as the keys are well-defined, which allows for
properly documenting them
* introduce ObjectType as an "enum"
* fix JSON marshalling and unmarshalling of EmailBodyStructure
* move the swagger documentation structs from groupware_api.go to
groupware_docs.go
* fix: change verb for /groupware/accounts/*/vacation from POST to PUT
