Add String method to ContextImpl to fix a race

[dnr]

What changed?
Add a String method (fmt.Stringer) to ContextImpl.

Also simplify contextMatcher implementation and move getOrCreateShardRequestMatcher to the bottom of the file just so the matchers are in one place.

Why?
This is a weird one: in some tests, we use a gomock matcher to differentiate between Contexts in CreateEngine, to return the right value. Internally, gomock evaluates all its matchers, but for the failing ones (and there will always be failing ones if there's more than one), it constructs an error message using Sprintf("%v") and then throws it away when it finds a match. The default "%v" formatter reads private fields of the value being matched. Some of those private fields (e.g. state) are protected by a mutex, but it doesn't know that and reads them anyway. This triggers the race detector. We can just provide a String to bypass the default "%v" behavior.

How did you test it?
ran test with -race -count 1000