libwebsockets

openssl: reuse client SSL_CTX where possible

If you have multiple vhosts with client contexts enabled, under
OpenSSL each one brings in the system cert bundle.

On libwebsockets.org, there are many vhosts and the waste adds up
to about 9MB of heap.

This patch makes a sha256 from the client context configuration, and
if a suitable client context already exists on another vhost, bumps
a refcount and reuses the client context.

In the case client contexts are configured differently, a new one
is created (and is available for reuse as well).