Expose Ssl option to user.
Merge pull request #126 from EasonLiao/ssl

netty transport: add ssl suport
netty transport: retry binding.

Travis build fails once in a while because it fails to bind to a port.
Retry binding to the port for 5 times before giving up in case this is
a transient issue.
Merge pull request #154 from m1ch1/master

netty transport: retry bind
Fix the bug of SendingReqTask on shutdown method.
Use different timeout for synchronizing peer.