* add bytelimit for prefetch
A fraction of byteLimit will be used as the limit to fetch index.
For the indexes fetched, fetch records for them in batch.
byteLimit always count the index size, it also count record if exist,
it at least return 1 index-record entry and always include the last entry
despite that adding the last entry despite it might exceed limit.
There is a Knob STRICTLY_ENFORCE_BYTE_LIMIT, when it is set, records
will be discarded once the byteLimit is hit, despite they are fetched.
Otherwise, return the whole batch.
* Add missing secondary queries tests for index prefetch
This change adds test for missing secondary queries for index prefetch,
in this case, MATCHED_ONLY mode would NOT return KV and UNMATCHED_ONLY
mode would return KV.
* remove default value for params
* Enable MATCHED and UNMATCHED mode for index prefetch
MATCHED mode returns index entries whose secondary KVs are present,
UNMATCHED mode returns index entries whose secondary KVs are absent.
Note that the conflict read range of this txn is set in 2 steps:
* Set the conflict range for primary query according to request
* Set the conflict ranges for secondary queries according to responses.
As a result, conflicts of different match_index mode are taken care of.
* Fix c binding
Markus Pilman