Support negative graph values.

Currently graph Y-Axes were hardcoded to start at 0. Choose the Y-scale
automatically based on the graph data instead.
Simple LevelDB sample dumper tool.

This very simple tool dumps all samples in a LevelDB metrics storage
together with their chunk information. This is very crude for now - I
just whipped it up because I needed to debug something. We should add
printf-style formatting support for it in the future to indicate what
information to output for each sample in the database. We could also
support inspecting the index databases.
Initial move away from skiplist.
Code Review: Initial arena size name.
Code Review: Avenues for future improvemnet noted.
Code Review: Extend lock scope for appending.
Code Review: Extend lock.
Code Review: Fix to-disk queue infinite growth.

We discovered a bug while manually testing this branch on a live
instance, whereby the to-disk queue was never actually dumped to
disk.