merkledb -- preallocate bytes.Buffer in codec

[danlaine]

Why this should be merged

May prevent doing some memory allocations

How this works

Self-explanatory

How this was tested

Existing UT

[joshua-kim]

Code looks correct to me but I'm just curious to see if we can get tighter bounds on the pre-allocation to see if we can avoid re-allocations later.

If we're concerned with the performance of our codec here we could also consider adding specific benchmarks tests for the codec to see if this is helping us.

[danlaine]

@joshua-kim Added estimates based on the number of children

[danlaine]

For what it's worth I expect this to be a pretty modest speedup. If we find that we're doing a ton of memory allocations here we can always bump the size estimates.