Add gokv.Store implementation for Hazelcast

[philippgille]

Hazelcast is advertised as IMDG (in-memory data grid), but at its core is an in-memory cache. We should add support for it to have another distributed cache option next to Memcached.

The supported data types are not key-value directly, but instead a single distributed map can be used for storing key-value pairs.

• https://github.com/hazelcast/hazelcast
• https://hazelcast.org/
• https://github.com/hazelcast/hazelcast-go-client
• https://godoc.org/github.com/hazelcast/hazelcast-go-client