leasedb: performance regression

[davidsarah]

The ​1818-leasedb branch has a performance regression that shows up when running the test suite -- in fact, the test suite is not able to pass at the moment purely due to timeouts.

Since the regression does not show up when using make tmpfstest (which uses a memory-based tmpfs filesystem rather than disk), my tentative conclusion is that it is due to the latency of leasedb database syncs. There are currently many redundant syncs due to every SQL query/update being in a separate transaction, and due to there being more SQL operations than necessary per request. We could also use a more relaxed consistency mode, if that is safe.