Block location Policy should not create for each file

[maobaolong]

Alluxio Version: Master branch

Describe the bug Block location Policy should not create for each file.

To Reproduce

Use roundrobin block location policy cannot round robin now, because the policy is created for each file and the index is stored within each policy object.

Expected behavior

Each policy should used for all files from a client.

Urgency

Urgency.

Are you planning to fix it

Yeah

Additional context No