Save object database to disk every X blocks when replaying

**User Story**

As a node admin I'd like that the node saves object database to disk every X blocks when replaying, so that it would save some time to replay again when something went wrong.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [ ] 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.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

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

No files, tests, configuration entry points, or exact checkpoint behavior are named. Start by tracing the node's block-replay path and object-database persistence, then determine how an administrator would set X and what happens after an interrupted replay. Done means the database is saved at the requested interval and replay behavior is covered by defined tests.

Written by the indexing model from the issue text.