Fix split-merge regression.

## Overview

This commit reverts [split-merge](https://github.com/boltdb/bolt/pull/181) and fixes the node.split() to do a multi-page split. This issue caused problems with bulk loading because it would split into a small page and a very large page. The very large page, in turn, would be an arbitrary size so when it was freed later it would be difficult to reuse and would cause serious fragmentation issues.

Reverting split-merge will cause poor utilization when randomly inserting with a high `DB.FillPercent`. My suggestion is to just use the default `DB.FillPercent` unless you know you'll only be doing sequential inserts.

## Notes

Thanks to @PreetamJinka for [reporting the issue](https://github.com/boltdb/bolt/issues/192) and thanks to @mkobetic for confirming through other testing.

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Fixes #196.

/cc @snormore
@mkobetic You're right. I originally tried keeping split-merge but that meant maintaining a dirty flag on each node but it was getting too complicated. I'll remove this later with a separate PR.