To enable RPC over SSL/TLS, specify server certificate and private key
using --rpc-certificate and --rpc-private-key options and enable
--rpc-secure option. After the encryption is enabled, use https and
wss scheme to access RPC server.
Introduced TLSContext that holds TLS related data that can be
shared with multiple SSL connections.
* src/DownloadEngineFactory.cc
* src/LibgnutlsTLSContext.cc
* src/LibgnutlsTLSContext.h
* src/LibsslTLSContext.cc
* src/LibsslTLSContext.h
* src/Makefile.am
* src/SocketCore.cc
* src/SocketCore.h
* src/TLSContext.h
* src/message.h