Update schema and document it to make it more understandable

[englehardt]

• I'm submitting a ... [ ] bug report [X] feature request [ ] question about the decisions made in the repository [ ] question about how to use this project

• Summary The current schema contains some patterns and names held over from OpenWPM's original instrumentation. These names aren't always meaningful in the new instrumentation. An example is the JavascriptCookies table, which should be renamed to Cookies as discussed here: https://github.com/mozilla/openwpm-webext-instrumentation/pull/11#discussion_r235051630.

In this bug we should first audit the current schema for all desired changes and land them.

[vringar]

Bumping this for multiple reasons:

• [x] new_request_id is apparently unused, so it should be removed
• [ ] req_call_stack is no longer used as there is a new table for that soon
• [ ] navigations.id is always null, because it's not set to autoincrement
• [ ] navigations.process_idalso seems to null(? need to verify)
• [ ] Remove the flash_cookies tables as Flash is effectively deprecated
• [x] Create a schema.md to document all relevant information

[vringar]

This is partially addressed in #556, so maybe we could make the effort to get all of this done in there.

[englehardt]

We should also move the documentation out of the wiki and into the repo in a docs folder, and update it to the new schema.