* protocols/plaintext: Move to stable futures and use unsigned varints
The plaintext 2.0 specification requires to use unsigned varints for
frame length delimiting instead of fixed 4 byte integer frame length
delimiting. This commit aligns the implementation with the
specification.
* protocols/secio: Fix doc comment BytesMut -> Vec<u8>
* protocols/plaintext: Add quick check smoke test
* protocols/plaintext: Rework imports and log levels
* protocols/plaintext: Use BytesMut instead of Vec<u8>
* protocols/plaintext: Use BoxFuture
