Allow full flush interval to be used for flushing with flush_jitter.

[danielnelson]

Feature Request

When using a flush_interval and flush_jitter of the same duration and the
random sleep jitter is close to the length of the interval, the output may
have no time available before the "Skipping a scheduled flush" warning is
emitted.

This forces the flush_jitter to be smaller than the interval making it
impossible to take advantage of the full interval period.

Current behavior:

If jitter is the same as interval and the flush takes "2s" you can expect
warnings if there is a high jitter followed by a low jitter:

[agent]
  flush_interval = "10s"
  flush_jitter = "10s"

If the jitter is half the interval then no warnings, but all writes take place
in the first half of the flush interval:

[agent]
  flush_interval = "10s"
  flush_jitter = "5s"

Desired behavior:

If a flush starts at the end of the period, there shouldn't be a warning
unless it exceeds a certain period of time.

Use case: [Why is this important (helps with prioritizing requests)]

Balancing connections to InfluxDB over the full flush interval without uneven
writes.