abi for eosio.system contract, draft of unit-test, get_blockchain_parameters API function, asset switched to signed int64_t, fixes

push elected producers to active producers, calculate parameters elected by producers, back to canceling unstake requests instead of decreasing

Fixed typo in line 33.  "give" to "given"

1. no longer need to specify index number
2. no longer need to specify type multiple times
3. clean up unnecessary references
4. remove member variables that are not needed
5. implement store for index
6. implement find iterator for secondary given primary key

(All tests other than identity_tests are working.)