SSL cleanup: moving some OpenSSL definitions to new dir folly/io/async/ssl

SSL cleanup: moving some OpenSSL definitions to new dir folly/io/async/ssl

Summary:SSLContext and AsyncSSLSocket are growing with a lot of code that is
OpenSSL-specific, and it may be good to refactor some of these before it gets
out of hand.

This is also useful to reduce complexity as we some additional features such as ServerHello parsing and TLS Cached Info (D2936570)

Main changes:
 * Created a subdirectory folly/io/async/ssl to refactor code from folly/io/async. We may want to consider moving this out of folly/io/async
 * Moved OpenSSLPtrTypes.h to folly/io/async/ssl/OpenSSLPtrTypes.h
 * Moved 'OpenSSLUtils' from SSLContext to separate file OpenSSLUtils.{h,cpp}
 * Moved TLSExtensions and ClientHelloInfo from AsyncSSLSocket to TLSDefinitions.h

Reviewed By: siyengar

Differential Revision: D2978707

fb-gh-sync-id: a21f02947aeffccc447da2124a91cc99315df1c7
shipit-source-id: a21f02947aeffccc447da2124a91cc99315df1c7