This commits removes the locks used to guard data integrity for the
database and replaces them with Transactions, turns out that SQL had
a way to deal with this all along.
This reduces the complexity we had with multiple locks that might stack
or recurse (database, nofitifer, mapper). All notifications and state
updates are now triggered _after_ a database change.
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
