api/asset: more efficient rebuildPool
api/asset: more efficient rebuildPool

As in 75676f21c15deba4bcdf17177569d4c539a2d39f, we do
not have to validate txs that are already in our pool.
We do have to check the inputs are satisfied, but that's
all.

Closes chain/chainprv#371.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain