DBAL-1106 Improve schema introspection performance on Oracle

[deeky666]

This PR improves the overall schema introspection performance on Oracle by optimizing some of the getList*() method SQL queries in the platform.
Unfortunately Oracle system views are rather slow, especially when joining tables. Using subqueries can improve the execution time a lot.

Improved queries

(the times have been measured using an exemplary query from the functional test suite)

getListTableForeignKeysSQL(): Down from ~2.33s to ~0.68s per query
getListTableColumnsSQL(): Down from ~0.49s to ~0.14s per query
getListTableIndexesSQL(): Down from ~0.5s to ~0.25s per query

Overall improvement of OracleSchemaManagerTest from ~100s to ~37.5s.

I was quite impressed by the performance improvements :)

[doctrinebot]

Hello,

thank you for creating this pull request. I have automatically opened an issue
on our Jira Bug Tracker for you. See the issue link:

http://www.doctrine-project.org/jira/browse/DBAL-1106

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included in.

[Ocramius]

Looks good, but will only land in 2.6 due to the risk of regressions

[deeky666]

Thx. Did not intend to backport it either :)