[serer][DBAgentMySQL] Add a feature to retry on the failure of communition between a MySQL server.

NOTE:
We still have the mysql functions we should retry such as
mysql_store_result().
[server][DBAgentMySQL] Show a message when a retry is succeeded.
[server][DBAgentMySQL] Close and connect again when communication with the MySQL server fails.
[server][DBAgentMySQL] Fix the message level.
[server][DBAgentMySQL] Don't reconnect in a transaction to keep a consistency.
[server][DBAgentMySQL] Set an internal flag for a connection state soon after calling mysql_close().
[server][CacheServiceDBClient] Define the maximum number of cache objects.
[server][CacheServiceDBClient] Define a structure to manage cache objects as a LRU list.
Merge branch 'issue-78'
Merge branch 'master' of github.com:project-hatohol/hatohol