Moneta: A unified interface for key/value stores
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Moneta provides a standard interface for interacting with various kinds of key/value stores.

Out of the box, it supports:

* File store for xattr
* Basic File Store
* Memcache store
* In-memory store
* The xattrs in a file system
* DataMapper
* S3
* Berkeley DB
* Redis
* SDBM
* Tokyo
* CouchDB

All stores support key expiration, but only memcache supports it natively. All other stores
emulate expiration.

The Moneta API is purposely extremely similar to the Hash API. In order so support an
identical API across stores, it does not support iteration or partial matches, but that
might come in a future release.

The API:

#initialize(options)::          options differs per-store, and is used to set up the store

#[](key)::                      retrieve a key. if the key is not available, return nil

#fetch(key, &block)::           retrieve a key. if the key is not available, execute the
                                block and return its return value.

#fetch(key, value)              retrieve a key. if the key is not available, return the value

#[]=(key, value)::              set a value for a key. if the key is already used, clobber it.
                                keys set using []= will never expire

#delete(key)::                  delete the key from the store and return the current value

#key?(key)::                    true if the key exists, false if it does not

#has_key?(key)::                alias for key?

#store(key, value, options)::   same as []=, but you can supply an :expires_in option,
                                which will specify a number of seconds before the key
                                should expire. In order to support the same features
                                across all stores, only full seconds are supported

#update_key(key, options)::     updates an existing key with a new :expires_in option.
                                if the key has already expired, it will not be updated.

#clear::                        clear all keys in this store