core/txdb: test reading/writing snapshot issuances
core/txdb: test reading/writing snapshot issuances

Add a unit test ensuring that writing a snapshot with issuance memory and
reading that snapshot back out preserves the issuance memory. There are
already analagous tests for the state tree.

Closes #741

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