Fix a bookkeeping error on watching wallet exports.
Fix a bookkeeping error on watching wallet exports.

The exported watching wallet would include all imported addresses'
chain indexes added to the chain index map.  Imported addresses are
special as they do not belong to the address chain, and this would
cause issues when serializing the wallet.

jrick/btcwallet,Stash-Crypto/btcwallet,Stash-Crypto/btcwallet,decred/dcrwallet,synrg-labs/btcwallet,jrick/dcrwallet,btcsuite/btcwallet,synrg-labs/btcwallet,jrick/dcrwallet,jrick/btcwallet,btcsuite/btcwallet,Roasbeef/btcwallet,decred/dcrwallet,Roasbeef/btcwallet