TLS / SSL thread safety with proactor

ssl_domain objects are semi-global.

For example two connections simultaneously creating or releasing their own private pn_ssl_t objects may mess up the refcount of the shared pn_ssl_domain_t object leading to memory corruption or leaks.

Windows schannel is further complicated by the OS internal refcounting of its security context thingies. That may get automatically solved by the above, or may require a separate JIRA to track. The same may apply to openssl.