[server][DBClientConfig] Add a comment.
[server][DBClientConfig] Add a comment.
[server][DBAgent] Add new structures for the next-generation index creation mechanism.
[server][DBAgentSQLite3] Implement code to create indexes with a new way.
[server][DBAgentMySQL] Add missing breaks in a switch block.
[server][DBAgentMySQL] Add a new method: getIndexes().
[server][Helpers] Add '\n' to improve the readablity of the error message.
[server][DBAgentMySQL] Implement code that makes an index when keyType is SQL_KEY_IDX.
[server][DBAgentSQLite3] Use a method name that is similar to that of DBAgentMySQL.
[server][DBAgent] Extend a structure: DBAgent::IndexesDef to be practicable.
[server][DBClient] Upper the order of indexesDefArray.

We will register multi-column indexes with this field.
If this patch is not applied, we need to write many 'NULL' in the
definition.
[server][DBAgent] Define a new pure virtual method: fixupIndexes().
[server][DBAgentSQLite3] Add a new method: getIndexes().
[server][DBAgent] Use a singular.

IndexesDef ->
IndexDef

The structure has information about one index. So the previous
name is not natural.
[server][DBAgentMySQL3] Implement makeCreateIndexStatement().
[server][testDBAgent] Test the case for non-UNIQUE index.
[server][testDBAgent] Define a pure virtual method: execSql().
[server][testDBAgent] Add a new method: createIndex().
[server][testDBAgent] Make createIndex() protected, because it's unncessary to be called from other than subclasses.
[server][testDBAgent] Define a pure virtual method: makeDropIndexStatement().