Enforce session advertised limits

We advertize a bunch of limits in the session:

    "urn:ietf:params:jmap:core" : {
      "maxConcurrentUpload" : 4,
      "maxSizeRequest" : 10000000,
      "maxConcurrentRequests" : 4,
      "maxCallsInRequest" : 16,
      "maxObjectsInGet" : 500,
      "maxObjectsInSet" : 500
    },
    "urn:ietf:params:jmap:mail" : {
      "maxMailboxesPerEmail" : 10000000,
      "maxMailboxDepth" : null,
      "maxSizeAttachmentsPerEmail" : 20000000
    }

• maxMailboxesPerEmail can likely be null if not enforced

Regarding maxConcurrentUpload and maxConcurrentRequests enforcing these limitations in a distributed environment is a non trivial task... Maybe via the use of external API management solutions? In which case we should allow an administrator to configure those values according to system wide setting...

maxSizeRequest, maxCallsInRequest, maxObjectsInGet, maxObjectsInSet, maxSizeAttachmentsPerEmail would likely only need some simple validation logic to be written.