Add initial value support to MemCacheStore increment and decrement calls

[andrejbl]

Summary

ActiveSupport::Cache::MemCacheStore cache store increment and decrement calls do not have support for setting an initial value on first call even though that is supported by the Dalli client underneath: https://github.com/petergoldstein/dalli/blob/5588d98f79eb04a9abcaeeff3263e08f93468b30/lib/dalli/protocol/binary.rb#L111

This change adds the ability to use the :initial option to set one.

Before this change, to set and increment a counter one needed to do the following:

Rails.cache.write("bar", 1, raw: true)
=> 5321567009203290112
Rails.cache.increment("bar", 1)
=> 2
Rails.cache.write("foo", 1, raw: true)
=> 12022923254730588160
Rails.cache.decrement("foo", 1)
=> 0

After the change, you can combine the two:

Rails.cache.increment("bar", 1, initial: 1) # if key doesn't exist set it to initial value
=> 1
Rails.cache.decrement("foo", 1, initial: 2)
=> 2

or call the operations without doing the write at all and it will assume the initial value from the amount (for increment) or 0 (for decrement):

Rails.cache.increment("bar", 2)
=> 2
Rails.cache.decrement("foo", 1)
=> 0

[eugeneius]

This needs a changelog entry, since we're changing the default behaviour when the key is missing.

We should make the same change to decrement, although I think the default value for initial there should be 0 rather than amount, so that both methods behave as if the key was set to 0 before they were called:

https://github.com/memcached/memcached/wiki/BinaryProtocolRevamped#increment-decrement

Decrementing a counter will never result in a "negative value" (or cause the counter to "wrap"). instead the counter is set to 0.