Use memory footprint of sugardb.store to compare against max memory for eviction policies

[osteensco]

Addresses #121

• Added memUsed field to SugarDB struct.
• Added GetMem() method to KeyData struct.
• Added GetMem() methods to Set and SortedSet structs.
• Added CompositeType interface to constants. CompositeType are types that implement the GetMem() method.
• Added MemoryUsed and MaxMemory fields to ServerInfo struct.

So I spent a good amount of time trying to understand the nuances with various data types in go and the amount of memory they take up. I think I've come up with a strong enough solution but I'm not sure any implementation would be perfect due to some limitations within go.

Some of the more primitive data types like int64 are straight forward in the amount of bytes occupied in memory, others not so much. Slices and strings have headers, structs will include padding between their various fields, and maps have a header consisting of a pointer, but also have several other components under the hood. I couldn't find an up to date method for accurately accounting for these other components since they have apparently changed over the years. So maps will be underreported, but my understanding is that the difference would be trivial for our use case anyway.

Open to feedback and happy to make adjustments or go a different direction on the implementation if needed.

[kelvinmwinuka]

Thanks for the work @osteensco. This solution looks good to me. I think due to some limitations in Go, we are limited in how accurately we can report mem usage anyways. We can always revisit this if we run into an issue in the future.

For now, I will approve and merge this.