Improve saving and noting changes workflow

[gterzian]

See discussion at #58 (comment)

The idea is to move the concept of last_heads: Vec<ChangeHash>, currently one per document, to a "per pending save per document" one, by moving it to DocState::Sync and DocState::PendingRemoval.

Processing would have happen at two places:

1. Inside note_changes, which would use the data found on the doc state instead of self.last_heads(this would remove the read lock by the way).

2. Inside save_document, which would use the last (fut, last_heads) pair to compute the next one.

[gterzian]

Not sure if the above would resolve what you meant as a potential data loss @alexjg

Should one perhaps rather keep adding saves for "longer" parts of the history that would include history for still pending saves, in the case previous saves are still pending?