[kairos] optimize pull nodes

- use epoch to determine if there is a result within the current
transaction

- have mux nodes produce lazier results

- eliminate Maybe indirection in transaction store for pull nodes

- separate out "mapping" from "filtering", allowing for more
fine-grained control over caching.

Flag: EXEMPT unused
Test: atest kairos-tests
Change-Id: I8ea329d40bca1e792cf38c96be444db202a91333