no need to fetch block in prev_txs for SequelStore

in wrap_tx blk_id is assigned only if block is in the main chain
more prev_txs cleanup

* for UtxoStore blk_id is always a block on the main chain
* for sequel store it stays the same as it was
* dummy only seems to care about the main chain
* for other storesi (to come) we assume blk_id will always be a block in the main
chain, which seems fair given Tx model only keeps association with one
block
fetch existing tx ids when storing block in one query instead of N
do one query instead of N to fetch existing addr ids
use copy_into for postgres inserts where it's simple enough
use COPY to store addr and tx for postgres
use COPY for massive inserts with postgres + cleanup
node: fix #on_getblocks when requested blocks are beyond our chain head