sessionctx: Error/warning on unsupported isolation levels

[morgo]

What problem does this PR solve?

Fixes #2712

What is changed and how it works?

For isolation levels, the following behavior is provided:

• read-uncommitted: error (unsupported)
• read-committed: clean
• repeatable-read: clean
• serializable: error (unsupported)

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Code changes

• minimal

Side effects

• Breaking backward compatibility (there may be some apps which depend on read-uncommitted or serializable, but they are less common than RR and RC)

Related changes

• Need to update the documentation
• Need to be included in the release note

---

This change is 

[morgo]

/run-all-tests

[zhouqiang-cl]

/rebuild

[morgo]

I have changed read-uncommitted to be an error now, and updated the description. read-committed also uses a new error class of NotRecommended.

[zhouqiang-cl]

/run-all-tests

[morgo]

It looks like this does break the JDBC test:

Loading JDBC driver 'com.mysql.jdbc.Driver'

Connected to 5.7.10-TiDB-v2.1.0-rc.3-299-g0dc57f7

[ERROR] Tests run: 30, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 4.957 s <<< FAILURE! - in pingcap.tidb.test.simple.ConnectionTest

[ERROR] testIsolationLevel(pingcap.tidb.test.simple.ConnectionTest)  Time elapsed: 0.01 s  <<< ERROR!

java.sql.SQLException: variable 'tx_isolation' does not yet support value: READ-UNCOMMITTED

	at pingcap.tidb.test.simple.ConnectionTest.testIsolationLevel(ConnectionTest.java:553)

This is the side-effect mentioned in the PR. Some applications will break on warnings too, particularly since read-committed is frequently used.

[morgo]

/run-all-tests

[morgo]

I was concerned that a warning for read-committed may break too many applications, so I've changed it to be clean again. The description has been updated.

[morgo]

/run-all-tests tidb-test=pr/683

[disksing]

LGTM.

[winkyao]

LGTM

[ngaut]

/run-all-tests