introduce new read-only transaction RPC end point

[linh2931]

Resolved #440

This PR introduces a new read-only transaction RPC end point. Major features are

• Provide a new endpoint send_read_only_transaction
• Provide a cleos push transaction option --read for pushing read-only transaction
• Provide a cleos push action option --read for pushing read-only action
• Do not sign any read-only transactions by cleos
• Do not allow any permissions for read-only transactions
• Assert any attempt to modify DB tables using DB API host functions
• Assert any attempt to modify DB tables using native functions
• Do not modify global_action_sequence and recv_sequence
• Do not do deepmind logging
• Add a catch-all flag in Chainbase to prevent any unlikely missed DB modification attempts.
• Add integration tests for cleos/chain_plugin/producer_plugin.
• Add unit tests for internal logic

Please note, the Chainbase submodule commit will be updated after corresponding Chainbase PR AntelopeIO/chainbase#4 is merged.

[greg7mdp]

Instead of repeating this pattern many times, why not create a new member function on transaction_context like:

deep_mind_handler* get_dm_logger() {
    return is_read_only() ? nullptr : control.get_deep_mind_logger();
}

[linh2931]

Thanks for your suggestion. Since read-only is an exception to deep-mind logging, it helps to understand the code by specifically showing when we don't do deep-mind logging.

[arhag]

I approved. But please fix the three remaining issues (two error strings, and the comments repeated at four different lines) before merging.