Allow setting the number of runtime workers to be used. (#258)

This change adds a new configuration parameter called `worker_threads` that
allows setting the number of workers the Tokio Runtime will use. It defaults to
4 to maintain backward compatibility.

Given that the config file parse is done asynchronously, first, a transient runtime
is created for reading config, and once it has been parsed, the actual runtime
that will be used for PgCat execution is created.

src/config.rs

@@ -178,6 +178,9 @@ pub struct General {
     #[serde(default = "General::default_ban_time")]
     pub ban_time: i64,
 
+    #[serde(default = "General::default_worker_threads")]
+    pub worker_threads: usize,
+
     #[serde(default)] // False
     pub autoreload: bool,
 
@@ -219,6 +222,10 @@ impl General {
     pub fn default_ban_time() -> i64 {
         60
     }
+
+    pub fn default_worker_threads() -> usize {
+        4
+    }
 }
 
 impl Default for General {
@@ -234,6 +241,7 @@ impl Default for General {
             healthcheck_timeout: Self::default_healthcheck_timeout(),
             healthcheck_delay: Self::default_healthcheck_delay(),
             ban_time: Self::default_ban_time(),
+            worker_threads: Self::default_worker_threads(),
             log_client_connections: false,
             log_client_disconnections: false,
             autoreload: false,