Log error when streaming failed
Fix timing problem; spec might fail
recorder: Add timeout for each storage on perform
fix typo
Add gracefully stopping via SIGTERM, INT
use Object#inspect
Allow storage to stop gracefully

Implement #exitable? and #stop! to stop gracefully.
Add AbstractStorage#name for logging
v0.2.0

= New feature

- Stop gracefully when received SIGTERM or SIGINT
  - Exit immediately if received twice

= Storage Development

- Allow storages to stop gracefully via #exitable? and #stop!
  - Akane::Manager calls storages' #stop! method when storages should
    stop
  - Storages should respond truthy value (e.g. `true`) for #exitable?
    when ready to quit

    Default implementation always return `true`

- Some log messages use #name method to identify Storage instance
  - Default implementation returns its class name