Bugfix/issue 27 (#29)

* Issue #27: Error with transactional pgv_insert/pgv_remove

* Issue #27: Error with transactional pgv_insert/pgv_remove - review

* Issue #27: Add fix and improve tests.

* Issue #27: Fix for backend termination on transaction commit.

* Issue #27: Add fix for leaked hash_seq_search.

 Fix warning: leaked hash_seq_search scan for hash table.

* Issue #27: Add tests and comment.

* Issue #27: Add fix for leaked hash_seq_search for pgv_stats.

* Issue #27: Add fix for VALGRING failed test.

* Issue #27: Add fix for Travis Ci failed tests.

* Issue #27: Fix tests for pgv_stats on 9.5

* Fix some comments

Co-authored-by: Maxim Orlov <[email protected]>
Co-authored-by: Alexey Kondratov <[email protected]>

Author: 3 people