Update builder of MergedEntityMetadata, add documentation, and refactor

[ading1977]

This pull request introduces the following changes:

• Update go protocol to reflect recent changes in SupplyChain.proto. The SupplyChain.proto was recently updated to add a commodities_sold_metadata with the type CommoditySoldMeta. The CommoditySoldMeta type defines a list of fields to patch for a specific commodity which is needed because not all fields in a commodity should be patched during stitching. Compared to this, the original commoditiesSold only allows settings of commodity types, and the stitching process will patch all available fields in a commodity sold.
• Add builder for CommoditySoldMeta in MergedEntityMetadata.
• Deprecate the original commoditiesSold field and the corresponding PatchSold and PatchSoldList functions.
• Add proper documentations to all exported functions.
• Refactor code.

[coveralls]

Coverage decreased (-2.06%) to 63.628% when pulling c49de35 on ading1977:OM-46988 into 572d0ca on turbonomic:master.

[pallavidn]

Fix this, then commit.
Default behavior of the keepStandAlone field should be false. Which means after merging the proxy entity will be removed. Probes all explicitly specify if they want the proxy entities to stay after merging.