CI: use special environment var in GHA for Windows' PATH

The old 'addPath' is depreciated now. For more details see:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

tests: fix parallel_exec_with sync implementation

Since std::condition_variable::wait can wakeup spuriosly we need to
pass a predicate to wait.

Merge stable-1.9 into stable-1.10

Merge stable-1.8 into stable-1.9

Merge stable 1.7 into stable 1.8

Merge stable-1.6 into stable-1.7

docker: make build- and push-image scripts more verbose

Merge stable-1.5 into stable-1.6

otherwise it will still use found (most likely, newer) clang-format.

Update doxygen settings

concurrent_hash_map: make double to size_t cast explicit in defragment()

bump docker images versions

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

and make sure only indexes up to max_index are used.

fixes #854

Merge stable 1.9 into stable 1.10

get rid of old WA for vcpkg and use latest release instead

force removal of docs' dir since it might not exist yet

add stable-1.10 to valid-branches.sh