Fee schedule code cleanup and refactory

**Bug Description**

Code in fee_schedule.* :
* logically `get` should be const, there should be no need for a non-const `get` and a const `cget` function;
* `get` should not throw an exception, if there is no fee param for an operation set in the fee schedule, the default fee param should be returned;
* `get` functions for individual operations are only needed if their logic is different from the default `get` template function;
* the code in `calc_fee_visitor` could be simplified if above items have been done.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. 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 Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation