Allow tuning buffers

- general buffer config
- add buffer, flushing, retry options in file buffer
- add flushing, retry options in mem buffer

Based on: https://docs.fluentd.org/configuration/buffer-section

Change-Id: I8fed7f97acc304c378ea3a5ba2d94304c5b39e1f
Related-bug: PROD-35697
3 files changed