Support settable value of socket timeout in mysql

[ykoyano]

This PR will fix #192.
I would like you to review my code.

[hito4t]

@ykoyano
Thank you for the PR!

As you know, embulk-output-jdbc sets different timeout value depending on retryableMetadataOperation.

retryableMetadataOperation is true in

• AbstractJdbcOutputPlugin#open -> INSERT statement will be executed (each statement will insert only one record, but multiple statements will be executed as batch)
• AbstractJdbcOutputPlugin#begin -> CREATE statement will be executed
• AbstractJdbcOutputPlugin#cleanup -> DROP statement will be executed

retryableMetadataOperation is false in

• AbstractJdbcOutputPlugin#commit -> COMMIT, INSERT statement will be executed (the INSERT statement will insert all records in intermediate tables)

Essentially, timeout for retryableMetadataOperation = true doesn't need to be large because DDL shouldn't take long time.
I think retryableMetadataOperation for AbstractJdbcOutputPlugin#open should be false because it will execute DML.

Would you tell me where the timeout occurred?
(I'd like to see stack trace for timeout exception.)