feat(net): add socket timeouts to Server and Client

While these methods are marked unstable in libstd, this is behind a
feature flag, `timeouts`. The Client and Server both have
`set_read_timeout` and `set_write_timeout` methods, that will affect all
connections with that entity.

BREAKING CHANGE: Any custom implementation of NetworkStream must now
  implement `set_read_timeout` and `set_write_timeout`, so those will
  break. Most users who only use the provided streams should work with
  no changes needed.

Closes #315

Cargo.toml

@@ -47,5 +47,5 @@ env_logger = "*"
 default = ["ssl"]
 ssl = ["openssl", "cookie/secure"]
 serde-serialization = ["serde"]
-nightly = []
-
+timeouts = []
+nightly = ["timeouts"]