vector: rm active tx assert in vector API

verify if non-const access methods/iterators works out of tx scope

it is unsafe to use some access methods/operators (e.g. at(), begin())
out of transaction scope, however we can't forbid users to use them, because there is no other way
to modify elements without transaction

test: cover missing vector cases based on codecov analysis

Add overload for array.range() method.

minor fixes to helper classes

This oveload takes only 2 parameters and snapshots the entire range
immediately. Return value is slice<pointer> instead of
slice<range_snapshotting_iterator<T>>. This change reduces overhead
of moving iterators in case where entire range is snapshotted.

Change array modifiers (assignment operators, swap and fill) behaviour.

Those methods now start internal transaction so that they are seen as
atomic from user's perspective. This behaviour is consistent with
pmem::vector.

Moreover those methods can be called only on pmem-resident objects.
Unlike in pmem::vector we cannot check if object is on pmem in
a constructor (because array is aggregate) so we postpone this to
a swap/fill/operator= function call. This check is performed only
in those functions bacause of performance overhead.

Remove string libcxx tests which won't be ported

refactor vector private methods

Add get(args) method overload for v.

These tests are either testing deduction guides (which won't be
implemented) or using custom allocator, which cannot be used with
pmem::obj::experimental::string.

Make vector's private methods more generic.
It will be beneficial for implementing vector's modifiers methods.

cmake: refactor the way we add tests to the project

get(args) method forwards args to the objects constructor.

operator= was only assuring that 'this' object was initialized by
calling get. This patch also assures that object from which we
assign is also initialized.

Run tests with PMEM_IS_PMEM_FORCE=1

Merge back changes from stable-1.5

Fix range_snapshotting iterator when snapshot_size == 0

When snapshot_size == 0, no snapshotting should happen. This condition
must be explicitly checked inside range_snapshotting_iterator.

Parameters to iterator should specify range accessible by this
iterator. Before this patch those arguments described the whole
array instead of just the part from start to start + n.

add support for situation, if we don't want to enable valgrind
tool A instrumentation, when test is ran with Valgrind tool B

Change implementation of canonical iterators for tests.

Update a docker image build-dependency.

It got transitioned to libunwind-dev in Oct 2014; there's still a compat
alias but using it gets you shouted at.

New version takes iterator type instead data type. This
allows for more flexible usage. For example it is required for
testing pmem::obj::experimental::basic_string where required
iterator type is const char*.

test: remove libcxx vector tests that won't be ported

Update Valgrind to 3.14

fix README list of flags