PLAT-4609 - Support Oracle RDBMS

Complete the main work to support Oracle RDBMS.

* Add SQL/DDL for position, portfolio, hts, convention and security masters
* Tidy up the existing code, reducing the need for dedicated Oracle SQL where possible.
* Fix data type assumptions that do not apply as Oracle prefers `BigDecimal` to `Long`
* Fix the dialect to not refer to HSQLDB
* Add to the README describing SQL differences

Given we're in an Oracle-specific file, do we know that this something we definitely have to deal with?