Refactor keypair retrieval and saving

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Issues found when working with network tests, keypair should not be automatically persisted when a keypair is not found. There was some janky logic that I wanted to clean up as well.
  • Essentially the problem is that a keypair is automatically pulled from keystore and used when generating new Libp2pService
• Cleaned up tests that unnecessarily heap allocated bytes and handled errors specifically in the test (found in Remove TODO rename old file to keypair.old(?) #217)

Reference issue to close (if applicable)

Closes

Other information and links