use added secret to protect convergent encryption

use added secret to protect convergent encryption

Now upload or encode methods take a required argument named "convergence" which can be either None, indicating no convergent encryption at all, or a string, which is the "added secret" to be mixed in to the content hash key.  If you want traditional convergent encryption behavior, set the added secret to be the empty string.

This patch also renames "content hash key" to "convergent encryption" in a argument names and variable names.  (A different and larger renaming is needed in order to clarify that Tahoe supports immutable files which are not encrypted content-hash-key a.k.a. convergent encryption.)

This patch also changes a few unit tests to use non-convergent encryption, because it doesn't matter for what they are testing and non-convergent encryption is slightly faster.