Replace the usage of a single application-wide session crypter
with per-session crypters.
The application is no longer able to decrypt any session
encrypted with its symmetric key alone. Instead, a session ticket
with its associated data encryption key (DEK) is also required in order
to decrypt the associated session data. The ticket itself is
encrypted with the application's crypter; the latter of which is
effectively a key-encryption key (KEK).
Fixes#49.
