Impl a few features & refactor some code based on design discussion.

Added async-trait as an optional dep. For now, included only when
postgres is enabled.

Extension traits are now being used for PgConnection, PgPoolConnection &
PgPool for listen/notify functionality. Only two extension traits were
introduced.

Only a single trait method is present on the extension traits and it
works for single or multi channel listening setups.

Automatic reconnect behavior is implemented for PgPool based listeners.
All logic has been cut over to the `recv` impls for the PgListener
variants.

Use async-stream for a nice Stream interface.

sqlx-core/Cargo.toml

@@ -15,7 +15,7 @@ authors = [
 [features]
 default = [ "runtime-async-std" ]
 unstable = []
-postgres = [ "md-5", "sha2", "base64", "sha-1", "rand", "hmac" ]
+postgres = [ "md-5", "sha2", "base64", "sha-1", "rand", "hmac", "async-trait" ]
 mysql = [ "sha-1", "sha2", "generic-array", "num-bigint", "base64", "digest", "rand" ]
 tls = [ "async-native-tls" ]
 runtime-async-std = [ "async-native-tls/runtime-async-std", "async-std" ]
@@ -24,6 +24,7 @@ runtime-tokio = [ "async-native-tls/runtime-tokio", "tokio" ]
 [dependencies]
 async-native-tls = { version = "0.3.2", default-features = false, optional = true }
 async-std = { version = "1.4.0", optional = true }
+async-trait = { version = "0.1.22", optional = true }
 tokio = { version = "0.2.9", default-features = false, features = [ "dns", "fs", "time", "tcp" ], optional = true }
 async-stream = { version = "0.2.0", default-features = false }
 base64 = { version = "0.11.0", default-features = false, optional = true, features = [ "std" ] }