update exchange, implement find for secondary index

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

Updated multi_index and new C DB API

Remove uint256 dependence from simpledb and abi_tests for now. Need to
later add support for key256 in ABI generation?

All tests except api_tests/transaction_test pass.

Dawn 417 restore release abi code generation

DAWN 646 - Fix problem where validation fails when creating new permission

DAWN-513 - Improve eosioc error message for 404 unknown endpoint

Tidy up http spacing

Makes error message more prominent

Fixes identity contract.
Only api_tests/transaction_tests fails.

grab max primary key in table on first call to available_primary_key
rather than on multi_index construction

Set minimum disk availability for install to 20 GB