MINOR: mux-h2: store the concurrent streams hard limit in the h2c

MINOR: mux-h2: store the concurrent streams hard limit in the h2c

The hard limit on the number of concurrent streams is currently
determined only by configuration and returned by
h2c_max_concurrent_streams(). However this doesn't permit to
change such settings on the fly without risking to break connections,
and it doesn't allow a connection to pick a different value, which
could be desirable for example to try to slow abuse down.

Let's store a copy of h2c_max_concurrent_streams() at connection
creation time into the h2c as streams_hard_limit. This inflates
the h2c size from 1324 to 1328 (0.3%) which is acceptable for the
expected benefits.