new filekv storage backend and related changes

[jessepeterson]

• New default storage backend: filekv
  • Deprecates the file backend; updated storage docs to reflect this.
• Similarly new storage backend: inmem
• Added e2e tests for push cert storage and migration.
• Fix in migration support in file storage backend: multiple TokenUpdate sending per device; found with new migration e2e test.
• Support sending of Bootstrap Tokens during migration from nano2nano: Support migrating bootstrap tokens #86
• Update NanoLIB version (bug fix for KV transaction wrapper for KeysPrefix).
• Misc. small tidy/clean-up and code re-org.
  • E.g. split primary storage interfaces into separate files.