sql: JDBC wants "set session parameters as ..."

[knz]

Using the same test: #3819 (comment)

Now fails with:

ERROR in (a-test) (QueryExecutorImpl.java:2182)
Uncaught exception, not in assertion.
expected: nil
  actual: org.postgresql.util.PSQLException: ERROR: syntax error at or near "AS"
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE
                            ^

 at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse (QueryExecutorImpl.java:2182)
    org.postgresql.core.v3.QueryExecutorImpl.processResults (QueryExecutorImpl.java:1911)
    org.postgresql.core.v3.QueryExecutorImpl.execute (QueryExecutorImpl.java:173)
    org.postgresql.jdbc2.AbstractJdbc2Statement.execute (AbstractJdbc2Statement.java:616)
    org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags (AbstractJdbc2Statement.java:452)
    org.postgresql.jdbc2.AbstractJdbc2Connection.execSQLUpdate (AbstractJdbc2Connection.java:399)
    org.postgresql.jdbc2.AbstractJdbc2Connection.setTransactionIsolation (AbstractJdbc2Connection.java:976)
    clojure.java.jdbc$db_transaction_STAR_.doInvoke (jdbc.clj:582)
    clojure.lang.RestFn.invoke (RestFn.java:464)
    jdbc_test.core_test/fn (core_test.clj:16)

[petermattis]

SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL is a verbose equivalent to SET DEFAULT_TRANSACTION_ISOLATION. Now that we support SHOW DEFAULT_TRANSACTION_ISOLATION we should probably also support SET.

[knz]

While looking at #4036