Merge stable 1.7 to stable 1.8

Merge stable-1.6 into stable-1.7

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

tests: cleanup source of removed test

Only parse libpmemobj version if it matches a pattern (major[.]minor.*)

doc: enable auto gh-pages generation in docker script

Ref: https://github.com/pmem/libpmemobj-cpp/issues/377

Add skip flags for additional artifacts built/installed in dockerimages

insert_erase_mt was removed but it's source stayed.

Merge stable-1.7 into stable-1.8

Merge stable-1.6 into stable-1.7

Create separate Docker images for stable-1.8 branch

Create separate Docker images for stable-1.7 branch

Create separate Docker images for stable-1.6 branch

Run find_gdb() in tests/CMakeLists.txt

It fixes regression introduced by bc3438ae.

detect GDB and disable tests

ref issue #469

Backport hash map erase fix to stable 1.8

Consider the following scenario (where A and B are in the same bucket):
thread 1:		thread 2:
find(acc1, A)
			erase(A)
find(acc2, B)

First, thread 1 takes lock on the bucket and then on element A.
Then, thread 2 takes a lock on the bucket and tries to lock element A
but it can't because thread 1 already locked it - this way thread 2 is blocked.
Next, thread 1 tries to lock the bucket but it can't because thread 2 holds it.
Neither thread can progress.

This patch fixes that by introducing the same algorithm as in lookup/insert.
When trying to lock element in erase we now spin in a loop doing try_acquire and
if we do not succeed after specified amount of time we drop bucket lock and try
whole operation again.

Merge stable 1.7 into stable 1.8

Fix overrding TEST_DIR variable