Camomile

- fix: fix broken 'keep_alive' feature

- feat: add 'keep_alive' login() parameter
- feat: add Camomile-specific exceptions
- feat: add 'file' getMetadata() parameter
- chore: improve code style

- feat: Python 3 support

- feat: {corpus | layer | medium} metadata
- feat: {corpus | layer | medium | queue } events subscription

- fix: fix a bug that crashes the server in metadata tree management

- fix: fix "watch" timeout for subsequent clients