Adding test for #1774 Sharding while keeping docs available

Test insures that writes can continue from a single writer while sharding happens on the table it is working on. Initially a known failure.
adding check that the expected data is present
refactoring the threads into a class, and moving that to utils
Merge branch 'larkost/1774-online-sharding-test' of https://github.com/rethinkdb/rethinkdb into larkost/1774-online-sharding-test