Implement API's to get withdraw_permission objects related to an account

[abitmore]

Listing some tasks need to be done in the core for #540 in this ticket, since that ticket may be mainly for discussion.

• query by giver
• query by recipient

[oxarbitrage]

i have a few options on how to this:

1- we make this as a plugin (recurring_plugin) and move the indexes of withdraw_permission_object into there just as account_history does. can have the option to save in elasticsearch database.
or
2- create a new recurring_api in api.cpp and don't move index.
or
3- just create the call needed in database_api.cpp

[abitmore]

Not like history objects, withdraw_permission objects need to be created in core. The by_expiration indices is being used in consensus. Other indices are not being used so far. If an index is useless in core, it's OK to have it in a plugin, but this may need some efforts, also don't know if we need to move it back to core for consensus which may get more work involved. Since RAM for this object type would not be a problem in a period, I suggest we just leave it in core. As a result, best add the API as a database API.

[oxarbitrage]

bitshares/bitshares-ui#30 - creating recurring payments in private testnet to test api calls as there is almost no public operations of this kind. ill extend a bit the documentation available with some simulated recurring payments examples as i was not able to find anything of value except for that link and https://bitshares.org/technology/recurring-and-scheduled-payments/ . Too little for such an important feature IMO.

[oxarbitrage]

closed by #676