Add invarient for threshold read.

Ensure the threshold read returns the threshold, and executes over
things which have already passed the threshold.
Update model to test threshold reads.

Test threshold read operations using the eqc_statem.  Verify that
operations on the reads only return the threshold specified;
precondition on the threshold not being available yet.
Add read and write concurrency to the table.

Add the read and write concurrency options on the ets table to see if
tha induces a failure when using parallel commands.
Ensure we compile code and tests.