Add a writeWalletDelimited/readWalletDelimited/isDelimitedWallet triplet.

The existing methods are not suitable for undelimited and padded streams. Note the delimited format is not compatible to the undelimited format. However, the only difference is a varint32 length field at the start of the message.

I put this PR on top of my other isWallet() PR. If you want to merge this first, let me know and I'll produce a version without the isWallet() methods.