feature: added undocumented API for attaching arbitrary key-value pai…

…r data to a cosocket TCP connecton, which can survive connection pool reuse.

This API may change in the future without notice.

bugfix: we now only recycle light threads which are "entry threads" o…

…therwise it is not safe to recycle user created threads since we have no idea whether the user may still hold refs to them.

bumped the API version to 0.10.18.

bumped version to 0.10.17.

updated .gitignore.