Remove abort rejection handler from ingest pipeline

We do not want to use the abort rejection handler on the batch writer.
This will discard writes if the incoming data starts to overwhelm the
batch writers, and we won't even be aware of it unless we look at the
logs.

We should use the default rejection handler which will still process the
data.
memoize computeShard

Part of calculating the md5sum is `MessageDigest.getInstance()`, which
is a blocking operation. Memoizing computeShard stops us from calling
this too often and is a massive speed up.
Merge pull request #332 from dlobue/memoize_computeshard

memoize computeShard
Merge pull request #333 from dlobue/remove_abort_rejectionhandler

Remove abort rejection handler from ingest pipeline