bug(merklesync-updater): tries to poll new_root before it has finished syncing all messages

[luketchang]

TL;DR the bug in the updater that failed all dev homes is as follows:

• Updater tries produce new_roots from a local merkle tree which is built from all the emitted messages for that home (same process as the processor)
• The updater has until now never synced/indexed messages
• The updater started syncing messages but before that has finished, it picks the current root of the local tree and uses that as its new_root
• This update of last_committed-->new_root is invalid (the new_root is actually a really old root)
• The improper update is submitted and the home is failed (for each updater)

Solution:

• Have the syncing merkle tree keep track of the current committed_root corresponding to its current root
• I.e. Each time the syncing tree stores a new leaf, it will get the Dispatch.committed_root field and update its current committed_root field
• The updater will only use tree.root() if tree.current_committed() == old_root