src/core. exec_ctx is now a thread_local pointer of type ExecCtx instead of
grpc_exec_ctx which is initialized whenever ExecCtx is instantiated. ExecCtx
also keeps track of the previous exec_ctx so that nesting of exec_ctx is
allowed. This means that there is only one exec_ctx being used at any
time. Also, grpc_exec_ctx_finish is called in the destructor of the
object, and the previous exec_ctx is restored to avoid breaking current
functionality. The code still explicitly calls grpc_exec_ctx_finish
because removing all such instances causes the code to break.
Useful for situations where we need to repeatedly trylock, not useful
for cases where we need to lock (due to spinning).
Add a variant of sync_test to test it (with the same tests we run for
gpr_mu).
Add a benchmark to bm_closure to demonstrate single threaded performance.