Memory leak in JsSetCurrentContext

Hi folks,

I've noticed that `JsSetCurrentContext` seems to leak small amounts of memory; see the screenshot below for a backtrace from Instruments on macOS. We're using ChakraCore in a server process, and during stress testing yesterday I noticed that we were leaking memory. Instruments shows that `JsSetCurrentContext` calls appear to be the cause; all roads lead to `ThreadContextTLSEntry` in the backtraces.

Is it possible that there's some required cleanup that I'm missing here? In our current design, request threads "borrow" a runtime from a pool, so we end up calling `JsSetCurrentContext` at least once per request. Are we supposed to "un-set" the context when a thread is finished using a runtime?