fix: vnode snapshot

refactor: adjust error codes

enh: delete redundancy code

docs: copy docs-examples from 2.6 to 3.0

test: case for alter stable

fix(query): set buffer page dirty when place data in it.

test(tmq): enable drop table case

docs: copy 2.6 documentation to 3.0 as the base

refactor: do some internal refactor, and set the correct intermediate buffer size for average function.

fix(query): ignore the check of qualified table id.

feat: dump sdb.data to json format

fix: retry if no enough shm

fix: unit test in json scalar operator

feat: refresh table meta based on sversion

feat(query): add nested query function

test:add testcase of   multi-Thread inserting data