autovacuum tuning with long transactions

[zadykian]

In the current implementation existence of long transactions is not taken into account during autovacuum-related parameters tuning.
This can cause useless or even harmful increasing of autovacuum intensity.
To deal with such cases, the oldest active transaction id should be logged with average bloat fraction.
And it case when bloat fraction is increasing, but the txid stays the same, autovacuum configuration shouldn't be changed.