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