Use PBKDF2 with HMAC-SHA1 for master passphrase in clawsrc.
The 64 bytes long key derivation is stored in 'master_passphrase'
pref, together with number of rounds used in its computation.
Introducing also two new common prefs:
master_passphrase_salt - holds a randomly generated 64 bytes
for use as salt with PBKDF2. Base64-encoded.
master_passphrase_pbkdf2_rounds - number of rounds (or
iterations) for next passphrase key derivation
The latter can be tweaked by user in case they want to use more
or less rounds, e.g. if they're running on weaker hardware and
KD with default number of rounds takes too long.