MINOR: pools: double the local pool cache size to 1 MB

MINOR: pools: double the local pool cache size to 1 MB

The reason is that H2 can already require 32 16kB buffers for the mux
output at once, which will deplete the local cache. Thus it makes sense
to go further to leave some time to other connection to release theirs.
In addition, the L2 cache on modern CPUs is already 1 MB, so this change
is welcome in any case.

(cherry picked from commit f587003fe916315e77e130339e21fc8d3a4a7248)
Signed-off-by: Willy Tarreau <w@1wt.eu>