debugger: wake up the event loop when a debugger command is dispatched
debugger: wake up the event loop when a debugger command is dispatched

When the event loop was blocked in epoll / kqueue or similar, debugger
commands wouldn't be processed. This patch fixes that by adding an
uv_async handle which is triggered when a debugger command is
dispatched. The async handle's callback makes sure that V8 is entered.

Closes GH-3626
Closes GH-3718

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node