no operation shows impacted both accounts when account is unproxied

hive::protocol::account_witness_proxy_operation contains empty string for proxy when account is unproxied, e.g.:

{"type":"account_witness_proxy_operation","value":{"account":"taco-shellinium","proxy":"keychain"}}
{"type":"account_witness_proxy_operation","value":{"account":"taco-shellinium","proxy":""}}

Because of this hafbe app must look back to most recent proxy op to get proxy value while processing blocks and thus is copying blockchain logic.

@bwrona suggested adding proxy_cleared_virtual_operation, which could have similar value to hive::protocol::account_witness_vote_operation:

{"type":"account_witness_proxy_operation","value":{"account":"taco-shellinium","proxy":"keychain","proxied":true}}
{"type":"account_witness_proxy_operation","value":{"account":"taco-shellinium","proxy":"keychain","proxied":false}}