CouchServerState is spread across test codebase and this option is
intended to be used only on server startup. This commit makes it default
to False and explicitly set it to True on where it's necessary.

Tests that were imported from u1db or created on top of that structure
were leaving temporary directories behind. This could cause problems in
test servers, either by filling the partition or by extrapolating the
maximum amount of files in a directory.

This commit replaces all usages of temporary directories in the old test
structure by pytest tmpdir fixture, which properly cares for removing
temporary directories.

We were using 'x'*size as payload, but on real usage the payload will be
random. This commit randomizes the payload using a predefined seed, so
the random payload will be the same across benchmarks.
Using random payloads also improves accuracy of compression or encoding
impacts and we will be evaluating those changes for resouce usage
issues.
Also note that base64 is used on payload. That was needed for utf8
safety, but overhead was removed to leave payloads as defined by
benchmarks.
Base64 was chosen also due its popular usage on MIME encoding, which is
used on mail attachments (our current scenario).

TestSyncEncrypterPool.test_encrypt_doc_and_get_it_back was trying to do
an operation and asserting the number of attempts. This test is about
putting a doc on encrypter pool and getting it encrypted. If we dont
wait for the encryption operation to succeed, then complex
trial-and-error happens, but if we just ask twisted to wait for one
operation before going to the other, this is not needed.

-- Resolves: #8398

This isnt a test, but a benchmark. Initialization sounds more like an
operation while instance is just something.

We are using lower values on test_encdecpool due high memory usage,
described in #7370. Added a comment to explain it.

defer parameter wasnt clear

Otherwise it will add unrelated overhead to results.