Separate validation and handling for recv slash packets

[shaspitz]

Description

This PR separates out validation and handling for slash packets received on the provider. This allows us to separate out the concerns of

1. At validation time, we return an IBC err ack immediately upon failure
2. At handling time, we stop handling (no-op), or panic the binary upon system critical failures like marshalling. No errors are returned

No functionality should be changed in this PR, but the refactors are needed to properly implement #462

Note: VSCMatured packet handling is refactored in this PR as well (just adds an explicit handler method). Jehan and I decided it doesn't make sense to have a validate method for VSCMatured packets since there's not a case in which to return an IBC err ack

Linked issues

Closes #414

Type of change

Please delete options that are not relevant.

• Non-breaking changes
• Updates in store keepers or store keys
• Testing

How was the feature tested?

• Unit tests
• E2E tests

Other information

Changes in functionality are reserved for #546 and #547

Checklist:

Please delete options that are not relevant.

• Relevant issus are linked
• Other PRs depend on this feature: #<issue>
• testutil/e2e/debug_test.go is up-to-date with additional or changed e2e test names

[jtremback]

LGTM except for the refactor getting rid of the getChainIdOrPanic method. But @mpoke really needs to review it.

[danwt]

Nice! This makes things much easier to read.

[mpoke]

Nice work @smarshall-spitzbart. See my comments below.

[mpoke]

LGTM