move tx_template to testenv and redo benchmarks

- refactor tx_template struct and init_graph
to use Cow type
- update the chain test_tx_graph test to use the
updated tx_template struct and init_graph fn
- update the test_tx_graph_conflicts to use the
updated tx_template and init_graph fn
- redo the canonical benchmarks to use tx_template

[Issue: 1754]

Author: tvpeter