[Discussion] Add ENABLE_TLS option

We already have the `ENABLE_SASL` option which is default OFF, and we used to regard that SSL module is a MUST HAVE.

But the SSL init in `net/security-context/tls-stream.cpp` will register Photon mutex, and will potentially cause core dumps if the app has used other third-party libs depends on SSL.

Since we can't remove all the third-party which might still be using std::threads, we sometimes need to disable SSL in Photon.

The new `ENABLE_TLS` option will only affect RPC client and HTTP client, and can defaults to ON.

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Another option is not to add this option, but to change RPC client API. If not passing a `TLSContext*`, the TLS stream will not be initialized, so the SSL global init will not be called.

Start by reading net/security-context/tls-stream.cpp and locating the existing ENABLE_SASL option, then inspect the RPC client and HTTP client entry points mentioned in the issue. Compare the proposed ENABLE_TLS option with the alternative TLSContext API change; the work is done only after one approach is selected and its effect on SSL initialization and both clients is verified.

Written by the indexing model from the issue text.