[PATCH][RFC] Various memory enhancements for MVT tiles

[taylor.smock]

JOSM start arugment: --download=39.0637818,-108.5670233,39.0660809,-108.5620022
Plugins: Mapillary (shift+, to download MVT tiles)

Memory allocations over 50s for r18430, limited to threads with protobuf related methods:

• Run 1: 8,979,920,352 bytes
• Run 2: 8,773,894,032 bytes

Memory allocations over 50s for r18431, limited to threads with protobuf related methods

• Run 1: 7,653,697,192 bytes
• Run 2: 7,133,589,400 bytes
• Run 3: 7,532,364,336 bytes

I'll go ahead and merge this patch on Monday, absent objections.

Memory allocations over 50s for r18431 + patch, limited to threads with protobuf related methods:

• Run 1: 823,974,256 bytes
• Run 2: 850,025,712 bytes
• Run 3: 756,248,936 bytes

[taylor.smock]

MVT performance improvements