RFR (XS): 8234127: BasicHashtable does not support small table_size

[Man Cao]

Thanks for the suggestion.
My patch for JDK-8087198 only uses the hashtable in fastdebug build for
correctness checking.
The patch is under review, and I'm not sure if we will eventually keep the
hashtable in codebase, though.
For G1UpdateBufferSize, I think 1 is actually quite useful for GC testing
purposes.

-Man