create signature collector and make processor stateless

[dknopik]

Issue Addressed

• Implemented as part of Validator Duties Service #27
• Improves upon Implement Anchor Processor #41

Proposed Changes

• Do not manage any longer running stuff in processor, as that provokes dependency cycles and weird locations for types.
• Create the signature collector, which... collects signatures. When all signature parts are received, we interpolate it into a full signature to be used for validator services. Currently, the actual cryptography of interpolating is mocked out.

Additional Info

The signature collector is the first component using the processor, which is why the small processor refactor is bundled with the signature collector in this PR.

[dknopik]

FYI: if this feels familiar to you, the changes are also included in #77, as this PR (more specifically the processor changes) is a prerequisite for #77. I'll rebase #77 once this is merged.

[jking-aus]

lgtm -- great work Daniel