Support MMI use cases

[shane-t]

MMI supports message signing, which means there are a couple of deviations from the normal chain of events that occur in MetaMask.

1. Messages can be signed several hours after they are approved depending on custody processes. To account for this, we propose a message status in addition to approved, signed called inProgress which unblocks the UI during this period. ([MMI] add setMsgInProgress method to AbstractMessageManager #1339)
2. Updates to messages in MMI come using custodian-internal identifiers, in addition to the metamaskId. We will add methods to set and retrieve this identifier.
   a. A method to arbitrarily set metadata (Support MMI use cases in signing typed messages #1364)
   b. A method to return all messages (rather than querying by identifier, in the client we will pull all messages, and then do a client-side interrogation of their content to get messages by custodyId (Support MMI use cases in signing typed messages #1364)
3. An event which is fired by the signature controller when a message ID is available, to allow certain MMI specific logic (adding message IDs to polling lists, and setting metadata (item 2a) (Support MMI use cases in signing typed messages #1364)
4. A means to defer setMessageStatusSigned by the addition of a boolean property to MessageParams (Support MMI use cases in signing typed messages #1364)

[shane-t]

Reopening due to a new requirement - we need to access the message manager methods we created, but we understand that the message manager is being encapsulated and should not be accessed directly.

For that reason two methods will be proposed for SignatureController, setMetadata and getMessages, both of which operate on all three managers.