wallet: Get rid of LockObject and UnlockObject calls in key.h

Replace these with vectors allocated from the secure allocator.

This avoids mlock syscall churn on stack pages, as well as makes
it possible to get rid of these functions.

Please review this commit and the previous one carefully that
no sizeof(vectortype) remains in the memcpys and memcmps usage
(ick!), and .data() or &vec[x] is used as appropriate instead of
&vec.