MerkleDB Improve Node Size Calculation

[dboehm-avalabs]

The node that is being stored in the cache contains more than just its bytes, so make the size calculation more accurate by tracking its total size.

[danlaine]

I think it might suffice to just change cacheEntrySize to:

return len(p) + 2*len(n.marshal())

It won't be exactly right but it'll be pretty close and the diff is 2 characters, which is nice.

[dboehm-avalabs]

I think it might suffice to just change cacheEntrySize to:

return len(p) + 2*len(n.marshal())

It won't be exactly right but it'll be pretty close and the diff is 2 characters, which is nice.

I'm cool with it if no one cares about it being 100% accurate.

[danlaine]

just nits