[DBClientUser] Rename a file name only.

This is one of steps to change the DB architecture.
[DBClientUsers] Change the class only.

DBClientUser ->
DBTablesUser

The logic is not changed at all.
[Param] Rename a constant: DB_DOMAIN_ID_USERS.

DB_DOMAIN_ID_USERS ->
DB_TABLES_ID_USER

This patch removes the last letter: S of USERS, because the class
name other definitions are also singular.
[testDataQueryContext] Update obsolete comments

DBClientConfig ->
DBTablesConfig

DBClientUser ->
DBTablesUser
[HatoholError] Update obsolete comments

DBClientConfig ->
DBTablesConfig

DBClientUser ->
DBTablesUser
Merge pull request #423 from project-hatohol/rename-dbtables-user-3

Rename DBClientUser to DBTablesUser