perf: reduce SQLite connection count and idle lifetime

On lightly loaded relays, we free up memory faster by letting idle
connections be reclaimed in 10 seconds instead of the default 10
minutes.  This also sets the minimum to zero connections, instead of
always trying to hold one open.

config.toml

@@ -42,7 +42,7 @@ description = "A newly created nostr-rs-relay.\n\nCustomize this with your own i
 # Database connection pool settings for subscribers:
 
 # Minimum number of SQLite reader connections
-#min_conn = 4
+#min_conn = 0
 
 # Maximum number of SQLite reader connections.  Recommend setting this
 # to approx the number of cores.