implement proxy connection pool

removeOldestProxyConnectionPool will be innefficient the larger the pool
is. A better data structure could be more efficient. Eg, make each value
in the pool include the timestamp of its oldest element, then the oldest
value can be found and modified, rather than rebuilding the whole Map.

But, for pools of a few hundred items, this should be fine. It's O(n*n log n)
or so.

Also, when more than 1 connection with the same pool key exists,
it's efficient even for larger pools, since removeOldestProxyConnectionPool
is not needed.

The default of 1 idle connection could perhaps be larger.. like the
number of jobs? Otoh, it seems good to ramp up and down the number of
connections, which does happen. With 1, there is at most one stale
connection, which might cause a request to fail.

doc/todo/git-annex_proxies.mdwn

@@ -32,9 +32,6 @@ Planned schedule of work:
 
 * test http server proxying with special remotes
 
-* http server proxying needs to reuse connections to special remotes,
-  keeping a pool of open ones. Question: How many to keep in the pool?
-
 * Make http server support clusters.
 
 * Support proxying to git remotes using annex+http urls.