Enable verification of write consistency level guardrails in
`modification_statement` and `batch_statement`.
Neither guardrail is enabled by default, so as not to disrupt clusters
that are currently using any of the CLs for writes. The warning
guardrail may seem harmless, as it only adds a warning to the CQL
response; however, enabling it can significantly increase network
traffic (as a warning message is added to each response) and also
decrease throughput due to additional allocations required to prepare
the warning. Therefore, both guardrails should be enabled with care.
The newly added `writes_per_consistency_level` metric, which is
incremented unconditionally, can help decide whether a guardrail can
be safely enabled in an existing cluster.
This commit adds additional `if` instructions on the critical path.
However, based on the `perf_simple_query` benchmark for writes,
the difference is marginal (~40 additional instructions, which is
a relative difference smaller than 0.001).
BEFORE:
```
291443.35 tps ( 53.3 allocs/op, 16.0 logallocs/op, 14.2 tasks/op, 48067 insns/op, 18885 cycles/op, 0 errors)
throughput:
mean= 289743.07 standard-deviation=6075.60
median= 291424.69 median-absolute-deviation=1702.56
maximum=292498.27 minimum=261920.06
instructions_per_op:
mean= 48072.30 standard-deviation=21.15
median= 48074.49 median-absolute-deviation=12.07
maximum=48119.87 minimum=48019.89
cpu_cycles_per_op:
mean= 18884.09 standard-deviation=56.43
median= 18877.33 median-absolute-deviation=14.71
maximum=19155.48 minimum=18821.57
```
AFTER:
```
290108.83 tps ( 53.3 allocs/op, 16.0 logallocs/op, 14.2 tasks/op, 48121 insns/op, 18988 cycles/op, 0 errors)
throughput:
mean= 289105.08 standard-deviation=3626.58
median= 290018.90 median-absolute-deviation=1072.25
maximum=291110.44 minimum=274669.98
instructions_per_op:
mean= 48117.57 standard-deviation=18.58
median= 48114.51 median-absolute-deviation=12.08
maximum=48162.18 minimum=48087.18
cpu_cycles_per_op:
mean= 18953.43 standard-deviation=28.76
median= 18945.82 median-absolute-deviation=20.84
maximum=19023.93 minimum=18916.46
```
Fixes: SCYLLADB-259
bb359b3b78