MB-62182: Implement File transfer APIs

[Thejas-bhat]

• Utilizes the new file transfer APIs to able to transfer the trained index file from one bleve index to another. This is helpful in a multi-partition scenario with a single datasource - where only one bleve index is trained on a set of vectors and then transfer the generated file to the other bleve indexes.
• The file transfer is currently focused around the trained index file but the protocol is such that its future extendable for other files if needed.
• The main control flow here is that the application needs to implement the IndexDirectory interface which maintains a reference to the destination bleve index.
  • Now, when the srcIndex.CopyFile(index.TrainedIndexFileName, directory) is invoked, it internally fetches the writer on the destination using directory.GetWriter()
  • Then, as part of the CopyFile it does an io.Copy() of the specific file and currently this is focused only for trained index file.
  • This is followed by an update bolt transaction + commit and sync in the destination index by invoking directory.SetPathInBolt(fileName, filePath). This is mandatory since bolt is reflective of the on-disk information.

[abhinavdangeti]

@Thejas-bhat this requires rebase and conflict resolution, also address any outstanding comments would you.

[coveralls]

coverage: 52.497% (-0.1%) from 52.61%
when pulling 7ac845f on fileTransfer
into 3d4b002 on master.