better HTTP connection reuse

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.

Assistant/Upgrade.hs

@@ -316,7 +316,8 @@ usingDistribution :: IO Bool
 usingDistribution = isJust <$> getEnv "GIT_ANNEX_STANDLONE_ENV"
 
 downloadDistributionInfo :: Assistant (Maybe GitAnnexDistribution)
-downloadDistributionInfo = Url.withUrlOptions $ \uo -> do
+downloadDistributionInfo = do
+	uo <- liftAnnex Url.getUrlOptions
 	gpgcmd <- liftAnnex $ gpgCmd <$> Annex.getGitConfig
 	liftIO $ withTmpDir "git-annex.tmp" $ \tmpdir -> do
 		let infof = tmpdir </> "info"