Use single curl multi handle to re-use connections

[jeroen]

This is a minimal proof of concept patch to test the performance implications of using a shared curl "multi-handle" to re-use http connections for multiple downloads to the same server (hence avoiding repeated TLS handshakes).

A more extensive patch, which includes cleaning up all connections when R quits, is available in #166

Quick benchmark:

options(repos=c(CRAN='https://cloud.r-project.org'))
x <- available.packages()
pkgs <- row.names(x)[1:100]
system.time(download.packages(pkgs, tempdir(), available = x))

Before:

   user  system elapsed
 15.212   1.113  24.098

After:

   user  system elapsed
  0.327   0.521   1.661

[sgsokol]

Nice update.
However, one point remains unclear for me: as curl_multi_cleanup(c->mhnd) is now commented out, how and when c->mhnd will be cleaned up?

[jeroen]

This is just to show the proof of concept. We would probably need to call curl_multi_cleanup when R exits.