polygon/heimdall: refactor into txReadStore and txStore

[taratorio]

Will be using NewTxReadStore in #10124

Idea is simple - split the current store in 2:

• txReadStore
• txStore

These 2 stores wrap around a tx with snapshots reader.
The txStore wraps around the txReadStore for code re-use.
This split removes chances of unexpected tx is read only errors when calling the store update methods.

Also, opens up a possibility to introduce a dbStore and dbReadStore wrappers around txStore and txReadStore if we decide to go down this path. The dbStore in particular can have ownership over the lifetime of a tester - make sure only 1 is open at a time for example by using a channel/mutex/semaphore or some other idea and call Rollback and/or Commit.

We may need to look into transitioning these to TemporalTx at some point - for now let's just get this to work first.