Refactor mailbox message formatting and add format checks

.../org/briarproject/bramble/mailbox/MailboxManagerImpl.java

...riarproject/bramble/mailbox/MailboxSyncRequestWriter.java

...briarproject/bramble/mailbox/protocol/MailboxMessage.java

...riarproject/bramble/mailbox/protocol/MailboxProtocol.java

...briarproject/bramble/mailbox/protocol/MailboxRequest.java

...arproject/bramble/mailbox/protocol/MailboxRequestEnd.java

...project/bramble/mailbox/protocol/MailboxRequestStore.java

...rproject/bramble/mailbox/protocol/MailboxRequestSync.java

...rproject/bramble/mailbox/protocol/MailboxRequestTake.java

...riarproject/bramble/mailbox/protocol/MailboxResponse.java

...mble/mailbox/protocol/MailboxProtocolIntegrationTest.java