Add mailbox props client, WIP

Mailbox-Properties-Client.md

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+
+# WORK IN PROGRESS
+
+The mailbox properties client is a [BSP client](https://code.briarproject.org/briar/briar-spec/blob/master/protocols/BSP.md) that synchronises mailbox properties between pairs of devices. Mailbox properties describe how to connect to a mailbox associated with a device.
+
+### Identifier
+
+The client's identifier is `org.briarproject.bramble.mailbox`. The major version is 0.
+
+### Groups
+
+The client uses a separate BSP group for communicating with each contact. The [group descriptor](https://code.briarproject.org/briar/briar-spec/blob/master/protocols/BSP.md#23-groups) is a [BDF list](https://code.briarproject.org/briar/briar-spec/blob/master/BDF.md) containing the unique IDs of the contacts' identities, sorted in ascending order as byte strings.
+
+### Message types
+
+**UPDATE** - The message body is a BDF list with two elements: `version` (int), and `properties` (dictionary). `version` is incremented whenever the properties change. The keys and values of the `properties` dictionary are strings. Typical keys in this dictionary are: *onionAddress*, *token*, *inboxId*, *outboxId*. An empty dictionary would mean for the receiver that there is no mailbox available for use.
+
+### Validity policy
+
+* An update is valid if it is well-formed.
+
+### Storage policy
+
+* In the groups shared with contacts:
+    * The local message with the highest version is stored.
+    * The remote message with the highest version is stored.
+
+### Sharing policy
+
+* In the groups shared with contacts, all local messages are shared.