feat: database reader connection pooling

Added connection pooling for queries, as well as basic configuration
options for min/max connections.

Cargo.lock

@@ -662,6 +662,8 @@ dependencies = [
  "lazy_static",
  "log",
  "nonzero_ext",
+ "r2d2",
+ "r2d2_sqlite",
  "rusqlite",
  "secp256k1",
  "serde 1.0.131",
@@ -811,6 +813,27 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "r2d2"
+version = "0.8.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "545c5bc2b880973c9c10e4067418407a0ccaa3091781d1671d46eb35107cb26f"
+dependencies = [
+ "log",
+ "parking_lot",
+ "scheduled-thread-pool",
+]
+
+[[package]]
+name = "r2d2_sqlite"
+version = "0.19.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "54ca3c9468a76fc2ad724c486a59682fc362efeac7b18d1c012958bc19f34800"
+dependencies = [
+ "r2d2",
+ "rusqlite",
+]
+
 [[package]]
 name = "rand"
 version = "0.6.5"
@@ -1028,6 +1051,15 @@ version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "254df5081ce98661a883445175e52efe99d1cb2a5552891d965d2f5d0cad1c16"
 
+[[package]]
+name = "scheduled-thread-pool"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc6f74fd1204073fa02d5d5d68bec8021be4c38690b61264b2fdb48083d0e7d7"
+dependencies = [
+ "parking_lot",
+]
+
 [[package]]
 name = "scopeguard"
 version = "1.1.0"