Do some connection pooling.

This prevents exhaustion of connections and also
has some performance benefits.

Cargo.lock

@@ -1185,6 +1185,7 @@ dependencies = [
  "downcast-rs",
  "itoa",
  "pq-sys",
+ "r2d2",
 ]
 
 [[package]]
@@ -2652,6 +2653,17 @@ version = "5.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
 
+[[package]]
+name = "r2d2"
+version = "0.8.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "51de85fb3fb6524929c8a2eb85e6b6d363de4e8c48f9e2c2eac4944abc181c93"
+dependencies = [
+ "log",
+ "parking_lot",
+ "scheduled-thread-pool",
+]
+
 [[package]]
 name = "rand"
 version = "0.9.4"
@@ -2994,6 +3006,15 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "scheduled-thread-pool"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3cbc66816425a074528352f5789333ecff06ca41b36b0b0efdfbb29edc391a19"
+dependencies = [
+ "parking_lot",
+]
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"