Follow-up patch.

This patch refactors CREATE/ALTER/DROP SERVER implementation:
- Use Sql_cmd subclasses to execute the statements.
- Reduce memory allocation by updating existing cache entries
  when executing ALTER TABLE (rather than replacing)
- Remove strlen() calls, use lengths from parser instead.
- Move code from sql_base.cc, sql_parse.cc, sql_lex.cc to
  sql_servers.cc