Correctly preserve metadata in `Transaction.newFromUnwrap`.

The `Transaction.pushReplace` method has a corner case if the removed
region has metadata and the inserted region is empty.  This works fine
unless there are two adjacent `pushReplace` operations, which can occur
in `Transaction.newFromUnwrap`.  Fix this by having `pushReplace` look
at a preceding replace and correctly merge the two operations if
possible (in particular in the tricky case where the previous case has
a zero-length insertion).  Pleasantly, this can be done without a lot of
special-casing code in `pushReplace` or `newFromUnwrap`.

Add test cases verifying the `newFromUnwrap` works correctly (both
in commit and in rollback) when there is metadata present.

Change-Id: I6cfec0d2b1823dad724422f018a3c73dc0c7f186
Remove no-insertion metadata corner case from `ve.dm.Transaction.pushReplace()`.

This version pushes a `replaceMetadata` operation after a `replace` to
fixup trailing metadata if there is no inserted region and the removed
region includes metadata.  This avoids a corner case where the
size of the metadata arrays inserted/removed in `Transaction.pushReplace()`
do not match the size of the data arrays inserted/removed.

We remove the now-unused `Document.getMetadataReplace()` method.

We also adjust `MetaList.onTransact()` so that it continues to work
properly when the number of metadata entries in `replace.insert` is
not the same as the number of metadata entries in `replace.remove`.

Change-Id: I1d600405b855ca1cb569853bb885b0752df47173