HPCC-9710 R embedding gives spurious stack errors in Roxie

The R code for checking stack limits assumes that all calls are on the same
thread as the original context was created on - this will not always be
true in ECL (and hardly ever true in Roxie).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4651 from richardkchapman/R-stack

HPCC-9710 R embedding gives spurious stack errors in Roxie

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>