Add Features: add MAdd and MGet

[bjkxt]

Update lru.go. Simple functions for many keys' operations with less mutex race.

[jefferai]

A change just went in that updates how eviction is handled -- can you update this PR against it?

[jefferai]

@bjkxt Got an approval from Brian, just looking for your changes against the new eviction code I think!

[bjkxt]

@bjkxt Got an approval from Brian, just looking for your changes against the new eviction code I think!

I think MAdd just takes the evictions of input keys and values to the caller. And the caller can decide how to handle these evictions.

[jefferai]

Two issues:

• There seem to be no tests for this functionality
• The code doesn't actually check whether the various slices are the same length, so it will panic if the caller provides bad input

[hashicorp-cla]

All committers have signed the CLA.