Handle case in MVCC of encountering a value written during same txn but
an earlier epoch. In this case, we want to read the original value
instead, so we scan for the next value prior to the intent.
Renamed scanNext to scanNextVersion.
Merge pull request #73 from cockroachdb/spencerkimball/txn-read-with-diff-epoch

Handle case in MVCC of encountering a value written during same txn but
Always read writes from same txn epoch.

We must do this even in the event that the timestamp of a write from our
own txn gets pushed forward so txn is attempting to read earlier
timestamp. We can't have a txn intent getting pushed forward by another
txn only to have it become invisible to the current txn, which isn't yet
aware the txn commit timestamp has been pushed.

Allow non-transactional reads and writes to push intents. This is
done by generating a random priority for the non-transactional actor.
We could consider allowing priority to be set as an optional field of
RequestHeader, outside of RequestHeader.Txn for non-transactional commands.

Change push txn code to always allow pushing the timestamp forward on
txn intents where isolation level is SNAPSHOT. Since snapshot isolation
txns don't mind having their timestamp advanced, this is better overall
for system performance--readers never block on snapshot isolation txns.

Added unittests for all cases.
Fixed error in commit 9ed5dbc.
Merge pull request #74 from cockroachdb/spencerkimball/always-read-txn-write

Always read writes from same txn epoch.
Update README.md and add new datastore comparison diagrams for scan
efficiency and read vs. write capabilities.
Merge pull request #76 from cockroachdb/spencerkimball/more-diagrams

Update README.md and add new datastore comparison diagrams for scan
updated readme with embedded link to Wired article; also updated ALPHA status.
Merge branch 'master' of https://github.com/cockroachdb/cockroach

* 'master' of https://github.com/cockroachdb/cockroach:
  Update README.md and add new datastore comparison diagrams for scan efficiency and read vs. write capabilities.
try a little harder to get embedded link to render correctly; might be impossible
try a little harder to get embedded link to render correctly; might be impossible
changed to simple image with link to embed preview of wired article
add InternalSplit command