Performance benchmarks can be found below. Obviously, a 8 KiB
request/response is tailored to showcase this improvement as this is
where codec buffer reuse shines, but I've run other benchmarks too (like
1-byte requests and responses) and there's no discernable impact on
performance.

We do not allow reuse of buffers when stat handlers or binlogs are
turned on. This is because those two may need access to the data and
payload even after the data has been written to the wire. In such cases,
we never return the data back to the pool.

A buffer reuse threshold of 1 KiB was determined after several
experiments. There's diminished returns when buffer reuse is enabled for
smaller messages (actually, a negative impact).

unary-networkMode_none-bufConn_false-keepalive_false-benchTime_40s-trace_false-latency_0s-kbps_0-MTU_0-maxConcurrentCalls_6-reqSize_8192B-respSize_8192B-compressor_off-channelz_false-preloader_false
               Title       Before        After Percentage
            TotalOps       839638       906223     7.93%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op    103788.29     80592.47   -22.35%
           Allocs/op       183.33       189.30     3.27%
             ReqT/op 1375662899.20 1484755763.20     7.93%
            RespT/op 1375662899.20 1484755763.20     7.93%
            50th-Lat    238.746µs    225.019µs    -5.75%
            90th-Lat    514.253µs    456.439µs   -11.24%
            99th-Lat    711.083µs    702.466µs    -1.21%
             Avg-Lat     285.45µs    264.456µs    -7.35%