Modify/posting pool (#1148)

don't return emptylist to pool
Switch posting list uids to a byte array.
add Metrics
change wal kv opt to table.nothing, posting kv to mmap
remove index and reverse log
add elog for eviction information
remove throttle for pending queries
set max concurrent streams to 1000
Fix hash tokenizer to use uint64 directly.
Replace lhmap with LRU cache. Just one for all groups.
round robin in client