Improve virtual ops storage, refactor account_history

[bitphage]

The problem: virtual ops now accessible only with enabled account_history plugin. So, without account_history get_ops_in_block API call just returns nothing.

So I propose the following thing:

1. Virtual ops storage to the database should be handled by separate plugin. virtual_ops for instance.
2. get_ops_in_block becomes a wrapper with the following logic:
   1. Whether account_history is disabled, it reads block content using get_block
   2. Whether account_history is enabled, it reads block content from account_history database
   3. Adds virtual ops using virtual_ops plugin