`operation_history_object::virtual_op` numbering may be discontinuous (possibly with holes)

**Bug Description**

In `database::push_applied_operation()`, after `_current_virtual_op` is assigned to `virtual_op` of the newly created `operation_history_object`, `_current_virtual_op++` is called: https://github.com/bitshares/bitshares-core/blob/f812666bb3bb884f6e8e5e7e669dc0050d3f3a69/libraries/chain/db_block.cpp#L535-L544

If the operation being pushed is in a proposal, after it is added to `_applied_ops`, it may be removed later if the proposal fails, but `_current_virtual_op` is not updated back: https://github.com/bitshares/bitshares-core/blob/f812666bb3bb884f6e8e5e7e669dc0050d3f3a69/libraries/chain/db_block.cpp#L361-L364

I'm not sure if it is worth fixing.

https://github.com/bitshares/bitshares-core/issues/1675 is related.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [x] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [x] UX (the User Experience)
- [ ] Other (please add below)

**Host Environment**
Please provide details about the host environment. Much of this information can be found running: `witness_node --version`.
- BitShares Version: `6.0.2`

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start with libraries/chain/db_block.cpp, especially database::push_applied_operation() and the proposal handling around the cited lines. Compare the virtual_op assignment and increment with the later removal from _applied_ops, then clarify the expected numbering behavior for failed proposals. Done means an agreed behavior is covered by a regression test and the API history no longer exhibits unintended holes.

Written by the indexing model from the issue text.