Queue watch changes

[alfonsogarciacaro]

Rigth now, Fable stops the watcher when it's compiling. Usually it's not a problem because watch compilations are fast, but if the compilation takes several seconds and users make some changes in the meanwhile these changes will go unnoticed. This PR adds a MailboxProcessor (actually, similar to Fable 2) so changes happening in the middle of a compilation are queued and not lost.

UPDATE: This PR now does a couple of things more:

• It removes the pseudo-cache of Fable compilation in Debug mode when there were already JS files and the F# sources hadn't been updated. I've realized the Fable compilation is needed to correctly build the watch dependency tree, so we cannot skip it.
• It goes back to @ncave's fork (actually a fork of a fork) because the FCS Nuget package had performance issues (discussion here) and also so we could keep fable-standalone compilation working.

[MangelMaxime]

Oh this could explain some weird behaviour I had sometimes where I needed to manually trigger watch to some files manually.