Require voted entity exists (issue #143)

[takaaki7]

This PR is work in progress, compilable but have not yet been tested.
I think this simple validation may prevent voting for non-existent entity. (#143)
And also we should check that all votes are unique.(account_options.votes is a flat_set so uniqueness check is unnecessary. )
May I go on with this plan?

[pmconrad]

I think the switch should operate on id.type()

[pmconrad]

Of course you may. Thank for taking up the task!

[takaaki7]

I've modified!

• Voted committees/witnesses counts are validated in account.cpp, so not validate here.
• To avoid duplicating hardfork number with hardforks created in previous repository, append "-2" suffix to issue number.
• Hardfork time is set at 08/28/2017 @ 12:00am (UTC).

Tested by using original func and vote for non-exist entities as follows.

signed_transaction vote_by_id(string voting_account,
                                        string vote_id,
                                        bool approve,
                                        bool broadcast /* = false */)
{ try {
      account_object voting_account_object = get_account(voting_account);
      voting_account_object.options.votes.insert(vote_id_type(vote_id));
      account_update_operation account_update_op;
      account_update_op.account = voting_account_object.id;
      account_update_op.new_options = voting_account_object.options;

      signed_transaction tx;
      tx.operations.push_back( account_update_op );
      set_operation_fees( tx, _remote_db->get_global_properties().parameters.current_fees);
      tx.validate();

      return sign_transaction( tx, broadcast );
} FC_CAPTURE_AND_RETHROW( (voting_account)(vote_id)(approve)(broadcast) ) }

unlocked >>> vote_by_id nathan "0:10" true true
vote_by_id nathan "0:10" true true
889304ms th_a       wallet.cpp:1908               sign_transaction     ] Caught exception while broadcasting tx 075f27c618eefbce33e149c15ea167f791d860c2:  0 exception: unspecified
Assert Exception: committee_idx.find(id) != committee_idx.end():

[pmconrad]

Good job, thanks!
Tested with HF date in the future - replays successfully
Tested with HF date before 02/2016 - replay fails
Tested with HF date in June 2016 - replays successfully

[pmconrad]

Please rename the constant to HARDFORK_CORE_143_TIME to make it clear that the number refers to the bitshares-core issue.

[takaaki7]

I'm sorry for being too late. I've fixed it.

[pmconrad]

No worries - thanks!

[oxarbitrage]

sorry for the delay. everything looks good, code looks good, test cases also good.
made the same tests as @pmconrad and getting the same results. approved.