Don't use cache before the initial sync is finished

fix sync setting cache ready (with test)

fix https://github.com/remotestorage/remotestorage.js/pull/509#discussion_r8013775

refactor https://github.com/remotestorage/remotestorage.js/pull/509#discussion_r8013810

self-explaining code https://github.com/remotestorage/remotestorage.js/pull/509#discussion_r8013807

remove put/delete that is irrelevant for this test https://github.com/remotestorage/remotestorage.js/pull/509#discussion_r8013816

 === instead of ==

queue get requests until the cache is ready for that path

avoid superfluous var declarations and correct array index

add regression test

use 'that' instead of 'this' from callback function