Run wal_checkpoint during housekeeping

[link2xt]

Otherwise WAL file may grow to 200M:
#6066 (comment)

Need to decide what type of wal_checkpoint to run, but probably TRUNCATE:
https://www.sqlite.org/pragma.html#pragma_wal_checkpoint

Before running it we should acquire all connections from the pool to make sure we don't run busy-handler. Then still better use block_on because some other process such as sqlitebrowser may open the database.