[Server][DBAgent] Prepare to create DB files with the given name (instread of DBAgentSQLite3-%d).
[server][DBClientSQLite3] Can specifiy arbitary database file name.
[server][DBClientSQLite3][test] Add assertions.
[server][DBClientZabbix] Can create a DBClientZabbix instance wihtout limitations of the server ID.

Previously, the server ID for the DBClientZabbix is limited.
However, now the other work on the branch issue-32 creates
a server ID with DB's auto increment feature. This implies
large number of server ID can be specified.
[server] Define ServerIdType to improve tolerability of its change.
[server] Save a DBAgentSQLite3 database files with a pretty name.

Ex.)

DBAgentSQLite3-32.db
-> hatohol.db

DBAgentSQLite3-4097.db
-> hatohol_cache_zabbix_0.db
[server][DBAgentSQLite3] Add getDBPath().
[server][test] Fix tests for the change of database names in 5f39c88a4.
[server][DBClientHatohol] Remove a wrong comment.

The implementation in the comment was changed at 04eec552.
[server][DBClient] Remove the unnecessary two-phase lookup mechanism.

Since f5257096, DBClientZabbix calls registerSetupInfo() on the
constructor. So this mechanism is no longer needed.
[server][DBClient] Improve performance.
[server][DBClient] Remove an unused variable.
[server] Add a 'const' qualifier, because it should be so.
[server][DBAgentSQLite3] Remove unnecessary call of defineDBPath().
[server][DBAgentSQLite3] Remove an unclear comment.
Merge branch 'dbclientzabbix-num-unlimited'