PendBuildRequest does not support cancellation

### Steps to reproduce

Manually working around lack of multiple nodes on Mono, I have come to this [solution](https://github.com/mono/monodevelop/blob/5d36aea55cb4174d8f9d94fd4c4d081a4e991348/main/src/core/MonoDevelop.Projects.Formats.MSBuild/MonoDevelop.Projects.Formats.MSBuild/ProjectBuilder.v4.0.cs#L233-L255)

This should help with parallel submissions by:
a) making a node's evaluation contents be reused
b) having submissions run in parallel

The problem is that there is no way to handle cancellation there. Seems like cancellation is built into node level, not submission level.

### Expected behavior
There should be a way to cancel a subsmission, before even running at at queue level.

### Actual behavior
There is no way to cancel it

### Environment data
```
msbuild /v
Microsoft (R) Build Engine version 15.7.222.58015 (xplat-master/9fe25dd0 Fri May 18 12:46:26 EDT 2018) for Mono
Copyright (C) Microsoft Corporation. All rights reserved.
```

OS info:
```
macOS
Mono JIT compiler version 5.14.0.105 (2018-04/6d28c8f9621 Fri May 25 04:53:26 EDT 2018)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: normal
SIGSEGV: altstack
Notification: kqueue
Architecture: amd64
Disabled: none
Misc: softdebug
Interpreter: yes
LLVM: yes(3.6.0svn-mono-master/8b1520c8aae)
GC: sgen (concurrent by default)
```

cc @cdmihai - this is the thing I was talking about.