prevent multiple websocket frames from being demanded in Jetty-12

[lachlan-roberts]

Jetty version(s)
12

Description
After PR #9945 you can no longer demand multiple frames at once.
We should remove the internal counter tracking demand and throw ISE or ReadPending exception if multiple frames are demanded.

ie. this code should be throwing ISE but instead it is just incrementing the internal demand counter.

for (long i = 0; i < n; i++)
           demand();