Use an exclusive lock on the DB dir while it's open

This prevents accidentally opening the same DB from multiple GumshoeDB
instances.

This will also prevent accidentally migrating on top of the same DB dir.
Add InsertUnpacked for inserting unpacked rows (with counts)
Gofmt
Add OpenDBDir to open a DB without providing a schema
[gumtool] Command 'migrate', to replace the migrator
[gumtool] Raise RLMIIT_NOFILE for migrations
[gumtool] Show migrate progress
More specific error when opening a non-existent DB
Store dimension table sizes with the metadata

This change also allows for loading empty dimension tables from
nonexistent files.
Add UntypedLess for comparing Untyped
[gumtool] 'stats', for printing out some DB info
Fix a bug where the RunConfig isn't being set
Test for rejecting overflowing inserts
[server] Show insert failures in statsd
[gumtool] When migrating, let the user control flush frequency

Keeping more of the data in ram rather than flushing after each segment
speeds up migrations by orders of magnitude.