chore(spooler): Change default connection pool to 1 connection (#3167)

This sets the sqlite connections pool to use max only 1 connection with
DB file.

Since we're processing only 1 message at a time right now, we do not
really need to have many connections open and maintained at the same
time. Which also will allow us to debug any weird looking issues
behaviour if that can happen.

relay-config/src/config.rs

@@ -823,12 +823,12 @@ fn spool_envelopes_max_disk_size() -> ByteSize {
 
 /// Default for min connections to keep open in the pool.
 fn spool_envelopes_min_connections() -> u32 {
-    10
+    1
 }
 
 /// Default for max connections to keep open in the pool.
 fn spool_envelopes_max_connections() -> u32 {
-    20
+    1
 }
 
 /// Default interval to unspool buffered envelopes, 100ms.