Checkpoint SQLite WAL off the request thread safely

Disable wal_autocheckpoint and run PASSIVE checkpoints from one elected
writer process. Contenders start in console/rake (initializer), Puma
workers (including WEB_CONCURRENCY=auto), and Resque children, with
failover, capped error backoff, and tests for lock takeover.

Co-authored-by: Thomas Klemm <github@tklemm.eu>

config/database.yml

@@ -9,6 +9,10 @@ default: &default
   pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 10 } %>
   timeout: 5000
   default_transaction_mode: immediate
+  # Checkpoint on a background connection instead (SqliteWalCheckpoint). Every
+  # non-test writer process runs a contender so this is never left without one.
+  pragmas:
+    wal_autocheckpoint: 0
 
 development:
   primary: