Serialize downloads of buildpacks

  - Increase inactivity timeout
  - Allow one buildpack downloader
  - Switch from parallel to serial download
  - Recover from failed downloads/unzips

[#72575882]

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>
Use ports instead of port, type and code are integers

[#73594384]

Signed-off-by: James Myers <jmyers@pivotallabs.com>
Add em-synchrony to Gemfile.lock