add "rebuild" param to utils' script, to force dockers rebuilding

it makes more sense to rebuild dockers all the time on stable branches,
because we don't have to count on DockerHub's policy to keep our
docker images on their servers.

Update doxygen settings

bump docker images versions

and set clang-format to fixed version (6.0).

fixes #854

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

fix dir creation in auto docs update

doc: enable auto gh-pages generation in docker script

allocator: remove unnecessary #include <libpmemobj.h>

Including libpmemobj.h causes inclusion of all type macros
which are not needed by C++.

[1.5] Do not check pkgconfig on windows

it might be installed (as part of cygwin).

Backport appveyor update to stable-1.5

... to reduce the Appveyor build time.

... to reduce the Appveyor build time.

PMEM_IS_PMEM_FORCE is not set in tests
and Appveyor build is timed out.

This change is required because of the following issue:
https://help.appveyor.com/discussions/problems/24684-problem-with-vcpkg-wrong-version-on-appveyor
https://github.com/appveyor/ci/issues/3052
which cannot be fixed directly now.

Extend warning message when valgrind is not found.

Install libpmemobj++.pc in all configurations

Specify we need devel package.

Adjust TRAVIS_COMMIT_RANGE variable to contain only commits from a PR.

to contain only commits from a PR.

Ref: travis-ci/travis-ci/issues/4596

[1.5] cmake: check if dot tool is installed

which is required by doxygen to generate graphs

[1.5] fix typo

Fix crash when previous transaction failed to start

... because of already taken lock.

Bump pmdk to 1.5.1 in Docker images.

Throw an exception when unlocking failed.