Validate connection information at Spider table creation

Spider allows a user to create a table with invalid connection information. For example, CREATE TABLE t (a INT) ENGINE=SPIDER is accepted by Spider but DMLs on the table will definitely result in errors. Note that we do not do a health check of data nodes at the table creation but do validation for obvious mistakes.

It might be good to have an internal switch for disabling the validation for the convenience of testing/QA.