This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
Unfortunately bcrypt only hashes the first 72 bytes of a given input
which meant that using it on ssh keys which are longer than 72 bytes
was incorrect.
This swaps over to using sha256 which should be adequate for the
purpose of protecting in memory passwords where the unencrypted
password is likely in memory too.
Before this change the race tests were taking too long. The bcrypt
function went from about 20ms to 1s under the race detector and this
is called for every transaction on webdav.
This change reduces the bcrypt strength so it takes 1ms non race so
the race tests pass and still has adequate security for in memory only
storage.
