Roadmap for API overhaul

[theoreticalbts]

There are many architectural issues in the API:

• Inconsistent pagination. Some API calls are paginated, some aren't. All API calls need to be checked for potential unlimited-size responses and proper pagination specifications designed and implemented.
• API calls should take and return structs, so adding additional arguments or returning additional information is possible in a reasonably backwards-compatible manner.
• Database API should be split up into several smaller API's.
• API's which require specific history plugins to be loaded should be placed into plugin-specific API's.
• No API's except the login API should be loaded by default.
• Web-specific stuff needs to be moved into its own set of plugins.
• Web-specific stuff needs to be made modular.

Here is a rough plan of action:

• Develop the new API's as an out-of-tree plugin set.
• When new API is tested and interface is stable, add submodules for core functions to tree, move old API's to out-of-tree "legacy" plugins.
• When all API clients (UI + exchanges + key third-party tools) have upgraded, stop maintaining legacy plugins.

[mvandeberg]

Related #209

[mvandeberg]

Related #1265

Some of the changes will satisfy this refactor.

[mvandeberg]

This is handled by Appbase