optimize: return setmetatable is NYI. (#30)

---- TRACE 162 abort setmetatable -- NYI: return to lower frame

doc: made it clearer that we depend on lua-resty-core.

bumped version to 0.07.

feature: added new method expire() that can change the TTL of the loc…

…k being held.

Signed-off-by: Yichun Zhang (agentzh) <[email protected]>

bumped version to 0.06.

optimize: use branch-free algorithms for variables assignment.

* the new step assignment combines both branches, and also ensures we do
not go over the value of `timeout`, so we avoid a third branch.
* we can avoid checking for step > timeout before the sleep if we take
care of it in `new()`. Further checks are done in the new step
assignment.

Signed-off-by: Yichun Zhang (agentzh) <[email protected]>

bumped version to 0.04.

bumped version to 0.03 and also updated docs to reflect recent changes.

bumped version to 0.02.