thrift-tls: Expose client common name in connection context
thrift-tls: Expose client common name in connection context

Summary:
Server can find out client's common name through
Cpp2ConnContext::getPeerCommonName

Test Plan: Added log line in the server method handler

Reviewed By: subodh@fb.com

Subscribers: jsedgwick, alandau, folly-diffs@, yfeldblum, chalfant, davejwatson, dancol

FB internal diff: D2469588

Tasks: 8331509

Signature: t1:2469588:1443658608:92d739db8445605db2092e67bcdfa175e3282a43

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