Limit job configurations run on travis.

fix doxygen invalid \ref command

fix clang compilation error

clang 7.0.0 triggers -Wself-assign-overloaded in case of explicit self assignment
whilst we handle this in our implementation

All words in the documentation that correspond to a documented
class and contain at least one non-lower case character will
automatically be replaced by a link to the page containing the
documentation of the class.

Merge back changes from stable-1.5

Add changelog for 1.6

Fix crash when previous transaction failed to start

string: cast null-terminator to proper type

... because of already taken lock.

Merge back changes from stable-1.5

test: add string tests for self assignment

to avoid Wsign-conversion.

test: enable -bigobj option for MSVC compiler

Fixes C1128 compiler error raised for some test binaries
due to exceeded limit of addressable sections

Add tests for various corner cases

fix make_persistent_atomic

string: add assign methods

fix rethrowing constructor exception

tests: change function name to match behaviour

add vector range() and crange() methods

Run tests on tmpfs on travis

fix string's methods description