www: add configurable limiters

Currently only for git-http-backend use, this allows limiting
the number of spawned processes per-inbox or by group, if there
are multiple large inboxes amidst a sea of small ones.

For example, a "big" repo limiter could be used for big inboxes:
which would be shared between multiple repos:

[limiter "big"]
	max = 4
[publicinbox "git"]
	address = git@vger.kernel.org
	mainrepo = /path/to/git.git
	; shared limiter with giant:
	httpbackendmax = big
[publicinbox "giant"]
	address = giant@project.org
	mainrepo = /path/to/giant.git
	; shared limiter with git:
	httpbackendmax = big

; This is a tiny inbox, use the default limiter with 32 slots:
[publicinbox "meta"]
	address = meta@public-inbox.org
	mainrepo = /path/to/meta.git

1 files changed, 1 insertions, 0 deletions

diff --git a/MANIFEST b/MANIFEST
index ceb1a9da..75bb43e3 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -106,6 +106,7 @@ t/cgi.t
 t/check-www-inbox.perl
 t/common.perl
 t/config.t
+t/config_limiter.t
 t/emergency.t
 t/fail-bin/spamc
 t/feed.t