agent: Always emit a readable callback when pseudo-TCP data arrives

agent: Always emit a readable callback when pseudo-TCP data arrives

Previously, the pseudo-TCP implementation’s readable() callback would
only be invoked when new data was received if it was enabled. The
callback is enabled by reading all data from the TCP input buffer until
EWOULDBLOCK is returned.

Reading all that data is not possible if the client buffer is of a
limited size, and can lead to livelocks if the client reads exactly the
number of bytes in the TCP input buffer (i.e. its buffer size matches
the TCP buffer fill level).

Instead, always invoke the readable() callback. This might be slightly
less performant, but the whole pseudo-TCP implementation is a shambles
anyway, and the callbacks need removing, so why not?