Summary:
This patch is groundwork for an upcoming change to store the set of
linked SSTs in `BlobFileMetaData`. With the current code, a new
`BlobFileMetaData` object is created each time a `VersionEdit` touches
a certain blob file. This is fine as long as these objects are lightweight
and cheap to create; however, with the addition of the linked SST set, it would
be very inefficient since the set would have to be copied over and over again.
Note that this is the same kind of problem that `VersionBuilder` is solving
w/r/t `Version`s and files, and we can apply the same solution; that is, we can
accumulate the changes in a different mutable object, and apply the delta in
one shot when the changes are committed. The patch does exactly that by
adding a new `BlobFileMetaDataDelta` class to `VersionBuilder`. In addition,
it turns the existing `GetBlobFileMetaData` helper into `IsBlobFileInVersion`
(which is fine since that's the only thing the method's clients care about now),
and adds a couple of helper methods that can create a `BlobFileMetaData`
object from the `BlobFileMetaData` in the base (if applicable) and the delta
when the `Version` is saved.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6835

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D21505187

Pulled By: ltamasi

fbshipit-source-id: d81a48c5f2ca7b79d7124c935332a6bcf3d5d988