Clean-up topology in meta repo when all indexes of a collection are dropped

Steps to repro:

a. Create a 1 node KV + n1ql + index cluster

b. Load travel sample

c. Drop travel sample

d. Query the localhost:9102/getLocalIndexMetada endpoint

The output contains topologies as under:

"topologies" : [

{

"bucket" : "travel-sample",

"version" : 8,

"scope" : "inventory",

"collection" : "airline"

},

{

"collection" : "_default",

"scope" : "_default",

"version" : 70,

"bucket" : "travel-sample"

},

This is a leak as the topology information is never created and the meta size grows. Also, in monitorKeyspace(), we read repo by topology. Since this topology exists in repo but it is actually deleted, we go and query ns_server thinking our cache is stale. ns_server says it is removed and we do nothing. 10 seconds later, same process happens - flooding ns_server with un-necessary requests