This patch ensures that all direct and indirect calls to
encryption_crypt provide a `dlen` value correctly initialized to the
destination buffer length, allowing encryption plugins to verify
available space. It also adds assertions to verify related invariants.
Signed-off-by: Trevor Gross <tmgross@umich.edu>
Instead of encrypt(src, dst, key, iv) that encrypts all
data in one go, now we have encrypt_init(key,iv),
encrypt_update(src,dst), and encrypt_finish(dst).
This also causes collateral changes in the internal my_crypt.cc
encryption functions and in the encryption service.
There are wrappers to provide the old all-at-once encryption
functionality. But binlog events are often written piecewise,
they'll need the new api.
