PostgreSqlSliceStore: log slice insertion time
PostgreSqlSliceStore: explicitly get block ID

When associating slices to block, pre-emptively query block ID and use
it, rather than a subquery, for slice insertions. While this requires a
round-trip to the database, it improves performance when inserting more
than ~20 slices by approximately 7% in the current djsopnet tests.
PostgreSqlSliceStore: bulk insert slices

Insert slices and associate them to blocks with one statement and query
each. Slice insertion is now 4-5x faster (around 1000 slices/sec on my
workstation).