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.

pgcat.toml

@@ -44,6 +44,9 @@ log_client_disconnections = false
 # Reload config automatically if it changes.
 autoreload = false
 
+# Number of worker threads the Runtime will use (4 by default).
+worker_threads = 5
+
 # TLS
 # tls_certificate = "server.cert"
 # tls_private_key = "server.key"