Use OpenSSL 1.1.0 ASYNC engine support

[nickm]

OpenSSL 1.1.0 will add a neat feature:

  *) Added ASYNC support. Libcrypto now includes the async sub-library to enable
     cryptographic operations to be performed asynchronously as long as an
     asynchronous capable engine is used. See the ASYNC_start_job() man page for
     further details. Libssl has also had this capability integrated with the
     introduction of the new mode SSL_MODE_ASYNC and associated error
     SSL_ERROR_WANT_ASYNC. See the SSL_CTX_set_mode() and SSL_get_error() man
     pages. This work was developed in partnership with Intel Corp.
     [Matt Caswell]

If this works the way I imagine it would, we could have OpenSSL stick the RSA/DH/ECDH operations for TLS in another thread (for performance) or even in another process (for isolation).

[nickm]

These tickets were tagged "6s194" as ideas for possible term projects for students in MIT subject 6.S194 spring 2016. I'm retagging with term-project-ideas, so that the students can use the 6s194 tag for tickets they're actually working on.

[teor]

Milestone renamed

[nickm]

Finally admitting that 0.3.??? was a euphemism for Tor: unspecified all along.

[nickm]

Remove an old triaging keyword.