Update unspentOutputs to remove spent unconfirmed outputs
Expose functions needed from the voting pool package
Address a couple of nits with recent En(De)crypt.

This commit adds comments about the specific crypto key types, moves the
selectCryptoKey function before the Encrypt/Decrypt functions that call it
to be more consistent with the rest of the code base, and slightly
modifies the verbiage of the comment.
Add the new ErrInvalidKeyType to the stringer map.

Also add it to the stringer test.
Voting Pools - Deposit side