[thread-safety] - BuildRequestEntry,TaskExecutionHost

Brief

Fix thread safety issues for the following classes, for the upcoming multi-threaded support.
Check parent issue for more details.
Depends on #11916

Areas

• BuildRequestEntry.cs

• TaskExecutionHost.cs

[JanProvaznik]

this ticket is too general, some parts were refactored in scope of other tickets, or the need for synchronization was not proven. Let's open specific issues instead as they arise.