Avoid locking the receive buffer when reading from the network

[Stebalien]

Currently, we lock the receive buffer when reading from the network. However, this means that if we're reading a large message, the receiving stream can't read anything already in the buffer.

A simple solution would be to allocate a temporary buffer for reading from the network, then copy into the receive buffer. However, that's an extra copy.

Instead, I'd prefer keep a read offset and a write offset into the same buffer, updating them after reading/writing. Critically, we wouldn't hold the lock while reading.