Parallel imap fetching w/fixes

@lightsofapollo Here is your https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/166 plus fixes to make things happy.  I tried to do a pull request against your branch, but I must have rebased or merged or something and the pull request ends up with 100 commits, which seemed bad.  So, uh, check my commits somehow, yo.