feat: TxnDatastore namespace and keytransform wrappers

[i-norden]

This PR introduces namespace and keytransform wrappers for the go-datastore TxnDatastore. This is in order to support a GetMany boxo blockstore implementation that leverages the TxnDatastore for batched reads which, in turn, is ultimately for supporting parallel traversal methods in go-hamt-ipld and go-amt-ipld.

boxo/blockstore PR that uses this: ipfs/boxo#492

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[hsanjuan]

@i-norden I don't see much has changed since last time?

[i-norden]

@i-norden I don't see much has changed since last time?

Ah apologies, I embedded but with a named field instead of anonymously so it still had the methods reimplemented but only to fall directly through. Fixed now!

[i-norden]

Also embedded ds.Txn into txWrapper, but since most the methods need to apply ConvertKey() the only ones this saved us from reimplementing are Commit() and Discard()

[hsanjuan]

@i-norden I'm not going to have much time in the next few days, but have a look to #211. I think it does what you want. You don't need to re-implement anything, you just need to wrap stuff in other stuff. If I haven't done something completely wrong, just take that, improving the naming of things and test it a bit.

[i-norden]

Thanks @hsanjuan! I appreciate the feedback. I'll take a look and test out that wrapper.

[lidel]

@i-norden is this PR still necessary or were you able to solve it with mentioned wrapper?