BUG/MAJOR: tcpcheck: Allocate input and output buffers from the buffer pool

BUG/MAJOR: tcpcheck: Allocate input and output buffers from the buffer pool

Historically, the input and output buffers of a check are allocated by hand
during the startup, with a specific size (not necessarily the same than
other buffers). But since the recent refactoring of the checks to rely
exclusively on the tcp-checks and to use the underlying mux layer, this part
is totally buggy. Indeed, because these buffers are now passed to a mux,
they maybe be swapped if a zero-copy is possible. In fact, for now it is
only possible in h2_rcv_buf(). Thus the bug concretely only exists if a h2
health-check is performed. But, it is a latent bug for other muxes.

Another problem is the size of these buffers. because it may differ for the
other buffer size, it might be source of bugs.

Finally, for configurations with hundreds of thousands of servers, having 2
buffers per check always allocated may be an issue.

To fix the bug, we now allocate these buffers when required using the buffer
pool. Thus not-running checks don't waste memory and muxes may swap them if
possible. The only drawback is the check buffers have now always the same
size than buffers used by the streams. This deprecates indirectly the
"tune.chksize" global option.

In addition, the http-check regtest have been update to perform some h2
health-checks.

Many thanks to @VigneshSP94 for its help on this bug.

This patch should solve the issue #936. It relies on the commit "MINOR:
tcpcheck: Don't handle anymore in-progress send rules in tcpcheck_main".
Both must be backport as far as 2.2.

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