Upgrade to cr-sqlite 0.16.0 + a few performance improvements

[jeromegn]

Upgrading to cr-sqlite 0.16.0 (unreleased, for now) gives a significant disk space reduction by adding a "lookaside" table for primary keys. Instead of repeating a whole primary key for each row in the clock table (there's 1 row per column + 1 sentinel row), this assigns an integer that can be looked up.

For consul services and checks, the savings are significant (about 30% in our case).

This PR also adds a few performance improvements:

• Process all "empty" (cleared) db versions out of band in bigger transactions
• Buffer more data when synchronizing change before sending them