Pass a parameter `IsExpr` from proxy to query node, so query 
node can create search plan according to the variable `IsExpr`.
It makes `segcore` unnecessary to distinguish `dsl` or `expr`
using dynamic exception. When `IsExpr` is set to true, query
node will call `CreatePlanByExpr` according to the passed
information about expression. Otherwise query node will keep
still to use `CreatePlan` according to the `dsl` information. At
the same time, this pr adds some unittests to `CreatePlanByExpr`,
these unittests translate already exist case with `dsl` to case with
`expr`.
Signed-off-by: Ndragondriver <jiquan.long@zilliz.com>