Publish storage functions latency to help compare the performance
of different disks in a single deployment.
e.g.:
```
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/1",server="localhost:9001"} 226
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/2",server="localhost:9002"} 1180
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/3",server="localhost:9003"} 1183
minio_node_disk_latency_us{api="storage.WalkDir",disk="/tmp/xl/4",server="localhost:9004"} 1625
```
- r.ulock was not locked when r.UsageCache was being modified
Bonus:
- simplify code by removing some unnecessary clone methods - we can
do this because go arrays are values (not pointers/references) that are
automatically copied on assignment.
- remove some unnecessary map allocation calls
Add a new Prometheus metric for bucket replication latency
e.g.:
minio_bucket_replication_latency_ns{
bucket="testbucket",
operation="upload",
range="LESS_THAN_1_MiB",
server="127.0.0.1:9001",
targetArn="arn:minio:replication::45da043c-14f5-4da4-9316-aba5f77bf730:testbucket"} 2.2015663e+07
Co-authored-by: Klaus Post <klauspost@gmail.com>
