Enable connection pool by default for H2 db

I encountered great difficulty reading data out of a CLOB field. 

Attempts to access the `JdbcClob` object, results in the following error:

```
JdbcSQLException The object is already closed [90007-174]
```

Enabling `:make-pool? true` would be a sensible default.

For more information about the issue: https://github.com/korma/Korma/issues/200