[XESh] Move all v8 related code to its own thread.

This patch fixes XESh so it can handle sync messages.
After some investigation we realized this bug was being
caused due to both native extensions code and their
JavaScript context being kept in the same thread.

This commit, thus, moves every v8 related code to its own thread.
XWalkExtensionClient was also moved to XEShV8Runner because it
relates to a v8::Context (through XWalkModuleSystem). That
ended up cleaning xesh_main.cc (hence, ExtensionManager), leaving
there only native-related pieces of the Extensions Framework.

BUG=https://crosswalk-project.org/jira/browse/XWALK-304
TEST=./xwalk/extensions/xesh/xesh_test.sh ./out/Release/