Fee schedule code cleanup and refactory

[abitmore]

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

[jmjatlanta]

Please take a look at 79a7abf and tell me if that is what you were thinking.

Some notes:

• account_create_operation does require a non-const reference to do some bit-shifting. Other options were considered, including a special function for just this object. But adding a mutable_get was the implementation I chose. It leaves the door open for similar things in the future, which could be considered a good or bad thing.
• if the fee_parameters do not exist, we attempt to 'do the right thing' by calling the correct get and adding that object to the flat_set.

The commit is untested. I will build some tests to verify this solution (especially the 'do the right thing' part).

A separate PR for that commit is doable if need-be.

[abitmore]

I left comments in the PR.
At this moment, I don't think it's the best time to work on this issue.