Add support for persistent Generation Host

[jeromelaban]

I'm submitting a...

• Feature request

Current behavior

The new Hosted Generation mode that uses out-of-process generation re-parses projects on each build, which can take a significant amont of build time.

It is important that a persistent process, such as "VBCSCompiler.exe" can stay in memory for as long as the msbuild processes that use it are running, so the parsed projects are kept in memory.

This feature is already implemented with the in-process model, via the reuse of AppDomains.