Replace r2d2 with an own connection pool

New connection pool does not use threads
and does not remove idle connections
or create new connections at runtime.

Cargo.toml

@@ -54,11 +54,11 @@ num_cpus = "1.15"
 num-derive = "0.3"
 num-traits = "0.2"
 once_cell = "1.17.0"
+parking_lot = "0.12"
 percent-encoding = "2.2"
 pgp = { version = "0.9", default-features = false }
 pretty_env_logger = { version = "0.4", optional = true }
 quick-xml = "0.27"
-r2d2 = "0.8"
 rand = "0.8"
 regex = "1.7"
 rusqlite = { version = "0.28", features = ["sqlcipher"] }