feat: add multiple group tsdb reads to table merge scan

feat: balance vgroup

test: add test case for tmq

feat(wal): provide is empty api

feat(stream): support retrieve

test: modify testcase of muti-mnodes

update abs for improve

feat: prepare for multiple group tsdb reads

feat: some sql syntax adjustments

feat(query): add irate function

test : add case for selective query about sample function 

enh: drop qnode and snode on dnode

feat: drop dnode

feat: add merge interval operator

fix(wal): return correct msg for open wal file