sql: add transaction_isolation default value

[erizocosmico]

Fixes #564

I added READ UNCOMMITED because it's what's most similar to what we do (no transactions at all).

READ UNCOMMITTED

SELECT statements are performed in a nonlocking fashion, but a possible earlier version of a row > might be used. Thus, using this isolation level, such reads are not consistent. This is also called a dirty read. Otherwise, this isolation level works like READ COMMITTED.

[kuba--]

Fix TestQueries/sequential/SHOW_VARIABLES

[erizocosmico]

Fixed @kuba--