BUG/MEDIUM: h3: Increase max number of headers when sending headers

BUG/MEDIUM: h3: Increase max number of headers when sending headers

In the same way than for the H2, the maximum number of headers that can be
encoded when headers are sent must be increased to match the limit imposed
when they are received.

Reasons are the sames. On receive path, the maximum number of headers
accepted must be higher than the configured limit to be able to handle
pseudo headers and cookies headers. On the sending path, the same limit must
be applied because the pseudo headers will consume some extra slots and the
cookie header could be splitted.

This patch should be backported as far as 2.6.