Ensure that we only have a single design-time build on open

[davkean]

Today, we have a bunch of components hooking up to JointRuleSource and ProjectBuildRuleSource to kick off design-time builds, typically when they are initialized. These trickle in, resulting in unneeded design-time builds during startup.

We should add a service that makes sure given a "normal" project gathers all the known design-time targets we're going to launch, listen to them and making sure we only have a single design-time build during startup.

[srivatsn]

As part of this something to consider: Should we also delay the build for CompileDesignTime until restore has completed instead of doing one and having to repeat it when restore completes?

[davkean]

@srivatsn Need to think about that a bit - that will delay the time it takes to get project-to-project references, rulesets, options, conditional compilation symbols, etc.