gcs: Start v2 module dev cycle.

[davecgh]

This removes the unused and undesired FromPBytes and FromNPBytes functions and associated tests from the gcs module in preparation for upcoming changes aimed to support new version filters for use in header commitments.

Since these changes, and several planned upcoming ones, constitute breaking pubic API changes, this bumps the major version of the gcs module, adds a replacement for gcs/v2 to the main module and updates all other modules to make use of it.

It also bumps the rpcclient module to v5 since it makes use of the gcs.Filter type in its API, adds a replacement for rpcclient/v5 to the main module and updates all other modules to make use of it.

Note that this also marks the start of a new approach towards handling module versioning between release cycles to reduce the maintenance burden.

The new approach is as follows.

Whenever a new breaking change to a module's API is introduced, the following will happen:

• Bump the major version in the go.mod of the affected module if not already done since the last release tag
• Add a replacement to the go.mod in the main module if not already done since the last release tag
• Update all imports in the repo to use the new major version as necessary
  • Make necessary modifications to allow all other modules to use the new version in the same commit
• Repeat the process for any other modules the require a new major as a result of consuming the new major(s)

Finally, once the repo is frozen for software release, all modules will be tagged in dependency order to stabilize them and all module replacements will be removed in order to ensure releases are only using
fully tagged and released code.