Enable HTTP connection reuse across multiple files, when git-annex
uses http-conduit. Before, a new Manager was created each time
Utility.Url used it. Now, a single Manager gets created the first time,
so connections are reused.
Doesn't help when external programs are used for url download,
but does speed up addurl --fast, fsck --from web, etc.
Testing fsck --fast --from web with 3 files, over high-latency
satellite internet, it sped up from 19.37s to 14.96s.
This commit was supported by the NSF-funded DataLad project.
9b98d3f630