Optimize block num returned on SyncNotes

[TomasArrachea]

There is a small optimization here where we return the block number we dropped, minus one. For example, if we match against blocks 1, 5, 10 and 15 but block 15 does not make it into the response, instead of returning 10 as the last included block, we return 14 (so the next query avoids blocks 10-14 because the user will start at 15 already). I doubt this is significant due to how the tables are probably indexed, so I would not bother doing it unless it's really trivial, but maybe it can be done in a follow up PR or issue

Proposed in #1843 (comment)