common/lru_map: rename tokens to entries

This code was originally used in a token cache, now
as a generic infrastructure rename token fields.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 532e41a9985a16b35a6e49cdcba38af0ad166fa8)
lru_map: don't use list::size()

replace list::size() with map::size(), which should have
a constant time complexity.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 7c1d2ded8fa8061bf3f14932800998b963745dd1)