use warn instead of puts

[ci skip]
Add expires support to mongo adapter.
mongo: use correct datatype date for expiresAt field
add mongo expiration test
readme: add backend feature matrix

[ci skip]
update readme

[ci skip]
update readme

[ci skip]
update travis.yml
readme update tdb

[ci skip]
readme update

[ci skip]
readme update

[ci skip]
fix mongo test case
mongo: add increment support
update readme

[ci skip]
readme

[ci skip]
update readme

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Revert "update readme"

This reverts commit ef3ef44d8d8ba9bf3c8a3602ab60b88b4e057711.
[ci skip]
readme: add footnotes

[ci skip]
readme update

[ci skip]
add IncrementSupport#convert_for_increment