Previously, hinted handoff had a hardcoded concurrency limit - at most
128 hints could be sent from a single shard at once. This commit makes
this limit configurable by adding a new configuration option:
`max_hinted_handoff_concurrency_per_shard`. This option can be updated
in runtime. Additionally, the default concurrency per shard is made
lower and is now 8.
The motivation for reducing the concurrency was to mitigate the negative
impact hints may have on performance of the receiving node due to them
not being properly isolated with respect to I/O.
Tests:
- unit(dev)
- dtest(hintedhandoff_additional_test.py)
Refs: #8624Closes#8646
de1679b1b9