[Feature]: skip signature verification when tx is retried in block-stm

[yihuang]

Summary

When tx execution is conflicted in block-stm, it get retried, in each retry, it runs exact same logic, but we can actually cache some stateless computations, signature verification is the most significant one.

Problem Definition

We can't cache it in normal transient stores, because they are cleared for each tx execution for deterministic execution results.

Proposed Feature

Add a field to Context just for this.