tarsum: start a test for TarSum

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: adding the layer for "scratch" image

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: test gofmt

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Add a mention of 80 column lines and reflow the document to hide the
evidence.

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
add UpdateSuffixarray and refactor TruncIndex

This commit refactors TruncIndex to make it possible to add container
ids to the TruncIndex without updating the Suffixarray.

This is useful during the Docker daemon's startup when we don't want to
update the Suffixarray for every container we add.

Add continues to function like before.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
update Suffixarray only once during daemon startup

This commit makes the Docker daemon call UpdateSuffixarray only after
it finishes registering all containers.

This lowers the amount of time required for the Docker daemon to start
up.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
tarsum: adding benchmarks

to cover a couple of use-cases:
* 1mb file, using no compression
* 1mb file, using compression
* 1024 1k files, using no compression
* 1024 1k files, using compression

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #5798 from unclejack/fix_startup_speed

don't update Suffixarray for every Register during startup
Merge pull request #5787 from SvenDowideit/pr_out_add_a_mention_of_80_column_lines_and_reflow_the_document_to_hide_the_evidence

Add a mention of 80 column lines and reflow the document to hide the evidence.
Merge pull request #5780 from vbatts/vbatts-start_tarsum_test

tarsum: start a test for TarSum