BUG/MEDIUM: httpclient/lua: crash when the lua task timeout before the httpclient

BUG/MEDIUM: httpclient/lua: crash when the lua task timeout before the httpclient

When the lua task finished  before the httpclient that are associated to
it, there is a risk that the httpclient try to task_wakeup() the lua
task which does not exist anymore.

To fix this issue the httpclient used in a lua task are stored in a
list, and the httpclient are destroyed at the end of the lua task.

Must be backported in 2.5 and 2.6.