The wallet password is defined as `bytes` in the rpc.proto spec and we are
passing it as a string, resulting in an incorrect password being set on a
new wallet and an inability to unlock an existing wallet.
Convert the wallet password to a Buffer before passing to `initWallet` and
`unlockWallet`, which is the default data type used to represent bytes in
the node grpc client. This is sures that the password is passed correctly
to lnd.
Fix#400
Use grpc.credentials.combineChannelCredentials to combine the ssl
credentials with the macaroons when initializing the lnd rcp client.
This avoids the need to pass the macaroon metadata throughout the
codebase.