feat(rpc):  return new error code TSDB_CODE_RPC_VGROUP_NOT_CONNECTED

enh: handle error while transfer snapshot

fix(vnd): save vnode info at the begining of vnode prepare commit

fix:remove assert

fix: restart snapshot sender on receiver is restart

fix(tdb/restore): seek jfd to begin

fix(query): show sql len on systable length modify from 1024 to 2048

refactor: do some internal refactor.

enh: enh chartset convert

fix: interval + fill desc order result different issue

fix: duplicated groups result in group by

fix: fill null value issue

fix(query): avg function support overflow sum value calculate

enh: print ipstr on taos_connect

test: add test cases

fix: skip msg

fix: keepColumnName parameter changes the user alias