[PATCH] Add log_transaction setting

Hi.

We encountered a problem with excessive logging when transaction is 
sampled.
When it is getting sampled every statement is logged, event SELECT. This 
can
lead to performance degradation and log polluting.
I have added new setting to filter statements when transaction is 
sampled - log_transaction.

Overview
===================
This parameter works very similar to log_statement, but it filters out 
statements only
in sampled transaction. It has same values as log_statement, access 
rights (only superuser),
setup in postgresql.conf or by superuser with SET.
But by default it has value "all" in compliance with existing behaviour 
(to log every statement).

Example usage
==================
Log every DDL, but only subset of other statements.

postgresql.conf
 > log_transaction = ddl
 > log_statement = all
 > log_transaction_sample_rate = 1
 > log_statement_sample_rate = 0.3

backend:
 > BEGIN;
 > CREATE TABLE t1(value text);
 > INSERT INTO t1(value) VALUES ('hello'), ('world');
 > SELECT * FROM t1;
 > DROP TABLE t1;
 > COMMIT;

logfile:
 > LOG:  duration: 6.465 ms  statement: create table t1(value text);
 > LOG:  statement: insert into t1(value) values ('hello'), ('world');
 > LOG:  duration: 6.457 ms  statement: drop table t1;

As you can see CREATE and DROP were logged with duration (because it is 
DDL), but
only INSERT was logged (without duration) - not SELECT.

Testing
===================
All tests are passed - default configuration is compatible with existing 
behaviour.
Honestly, I could not find any TAP/regress tests that would test logging 
- some
tests only use logs to ensure test results.
I did not find suitable place for such tests, so no tests provided

Implementation details
===================
I have modified signature of check_log_duration function - this accepts 
List of
statements that were executed. This is to decide whether we should log
current statement if transaction is sampled.
But this list can be empty, in that case we decide to log. NIL is passed 
only
in fast path, PARSE and BIND functions - by default these are logged
if transaction is sampled, so we are compliant with existing behaviour 
again.
In first implementation version, there was boolean flag (instead of 
List), but
it was replaced by List to defer determining (function call) whether it 
is worth logging.

-------------
Best regards, Sergey Solovev