Allow schema cache path to be defined in the database configuration file

[kytrinyx]

Summary

This updates the database tasks for dumping the Active Record schema cache as well as clearing the schema cache file, allowing the path to be defined in the database configuration YAML file.

As before, the value can also be defined in an ENV variable, though this would not work for a multi-db application. If the value is specified neither in the DB config, nor in the ENV, then the path will continue to be derived from the DB config spec_name.

Other Information

In order to make the schema cache database tasks more uniform, I added a clear_schema_cache method to the database tasks, in symmetry with the dump_schema_cache method. Then I pushed the filename logic into those methods, making it easier to test in a way that is understandable. Update: had to back that out, as it was a breaking API change

The schema cache dumping task was only very lightly tested, so I fleshed those tests out a bit, and the clearing task was not tested, so I added tests for those.

I didn't find any in-depth documentation for the schema cache or the database configuration in the Rails guides, so I only added a bit of method documentation on the new schema_cache_path accessor on the database config. If you can think of any other documentation that needs updating, please let me know!

[tenderlove]

I like that this allows us to specify the schema file in the db config. I think this is a güd change. (@kytrinyx didn't mention it, but we already do this at GitHub, so this is kind of upstreaming our changes.)

[rafaelfranca]

Nice work! We were going to apply a similar change as well. I made two comments about the CHANGELOG.

[eileencodes]

This looks great, can you squash your commits into one?

[eileencodes]

Oops I missed something when I reviewed before

[seejohnrun]

This PR looks good! I left a few comments around consistency with other HashConfig / DatabaseConfig method 🎉

[kytrinyx]

I've reworked this based on the discussions so far:

1. This reverts back to the original API of the dump_schema_cache method
2. It changes the API of the schema_dump_filename but it is backwards compatible
3. I've added the method with a raise NotImplementedError in the DatabaseConfig class
4. I decided not to change the parameter to schema_dump_filename to spec_name but rather keep the original, since we're no longer in a context of a DatabaseConfig object.
5. I tried moving the default name implementation into the HashConfig#schema_cache_path, but realized that it was too gross for the db config class to know about the rails top-level config, so I have left it in the DatabaseTasks.

Let me know if there are any other changes you can think of, and if not I'll squash all the commits.