refactor EOS VM OC's code cache to use serialized file

[spoonincode]

Currently EOS VM OC's code cache is implemented via a boost interprocess allocator that is mapped from a file (much like how chainbase works).

This is over-complication.

It should be refactored to simply de/serialize the code from/to disk on start/stop. This will incur a small performance impact at start/stop but if the serialized file is compressed I expect it to be only a minimal impact even when a slow disk is used. The benefits are substantial:

• No longer subject to boost version breakage
• Can eliminate the multi-process view of the cache cache
• Can eliminate the multi-process architecture needing to track lifecycle of main-process' OC code_cache instances
• nodeos data-dir can be on a noexec filesystem
• OC's code cache can be in huge pages
• It'll be easier to accommodate in-place upgrades
• Probably more