[Collections] scope/collection creation/drop fails with 503 status error

Summary:
Some Scope/collection creation/drop fails with 

"error: 503 reason: unknown "Operation is unsafe at this time. Retry later."
Unsure if this is something expected as with some sleep put (or retries later) to ensure these drops/creation happen after rebalance has started progress, it doesn't throw this error. 

Steps to Reproduce

1. Create a 3 node cluster
   2020-07-28 06:01:59,322 | test | INFO | pool-2-thread-7 | [table_view:display:72] Rebalance Overview
   -----------------------++-------------

   Nodes

   Services

   Status

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     2. Create buckets with collections and load data
2020-07-28 06:12:05,461 | test | INFO | MainThread | [table_view:display:72] Bucket statistics
-----------------+++-------------------------------+---------------------------------

-----------------+++-------------------------------+---------------------------------

-----------------+++-------------------------------+---------------------------------

   3.   Start rebalance-in op with CRUD on collections in parallel
Some CRUD on collections fail. Taking one such example:

2020-07-28 06:12:08,174 | infra | ERROR | Thread-1839 | [Rest_Connection:_http_request:205] DELETE http://172.23.105.211:8091/pools/default/buckets/bucket2/collections/_default/mA body: headers: {'Accept': '*/*', 'Connection': 'close', 'Authorization': 'Basic QWRtaW5pc3RyYXRvcjpwYXNzd29yZA==', 'Content-Type': 'application/x-www-form-urlencoded'} error: 503 reason: unknown "Operation is unsafe at this time. Retry later." auth: Administrator:password

2020-07-28 06:12:08,181 | test | ERROR | Thread-1839 | [bucket_ready_functions:drop_collection:814] Collection 'bucket2:_default:mA' delete failed: "Operation is unsafe at this time. Retry later."

Rebalance starts progress at:
2020-07-28 06:12:12,269 | test | INFO | pool-2-thread-20 | [table_view:display:72] Rebalance Overview
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Have attached the consoleText. Screenshot of UI showing collection mA still present. Have attached the cb-collect-info.