Add more GSI stats to UI and periodic stats

Consider adding stats like below to UI as well as periodic stats. Need further pruning to come with useful stats list.

Projector CPU/Memory
Mutation Queued docs vs Pending docs vs Flush queued docs
Disk IO stats
Warmup progress
Number of outstanding scans
Number of scan timeouts/scan errors
Largest key size
JEMALLOC stats
End-to-end scan latency (including GsiClient)