Add flush_every_n_blocks config option to BufWriter generators

[TheSafo]

What does this PR do?

Adds flush_every_n_blocks to traditional and logrotate generators. This is designed to be used in conjunction with block-based throttling to ensure that blocks are still flushed regularly.

Motivation

BufWriter generally only writes out to disk once its capacity is full (or on log rotate). For standard usage of lading, this works fine since the capacity is set at the maximum throttler capacity (in bytes), which lading trires its best to run at, so it flushes ~every second. However, for block-based throttling (and varying block sizes) it's less likely that 1 second of blocks fills the maximum possible bytes capacity for those blocks (blocks per second times max block size), so having a config option to manually flush every few blocks ensures regular flush intervals.

Related issues

Follow up of #1675

[TheSafo]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• static-timestamped #1687
• Add flush_every_n_blocks config option to BufWriter generators #1683 👈 (View in Graphite)
• Update generator throttler to support block-based strategy #1675
• Add deserialization tests for BytesThrottleConfig and LoadProfile #1684
• Modify throttle errors to not assume capacity issue #1679
• Resize block buffer after allocation if significant wasted space #1674
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