Check for freelist overflow.
Fix deadlock on remmap.

This commit fixes a deadlock situation that can occur when Bolt's database size crosses a threshold
and requires remapping the mmap.
Merge pull request #194 from benbjohnson/fix-deadlock

Fix deadlock on remmap.
Fix freelist rollback.
Merge pull request #193 from benbjohnson/check-freelist-overflow

Check for freelist overflow
Document key/value lifecycle.

This commit adds a caveat to the godoc that explains how key and value
byte slices will only be valid for the life of the transaction.
add freelist stats to db stats
drop mergeStats and move freelist stats update to Tx
Merge pull request #197 from Shopify/freelist_stats

Add freelist stats to db stats
Add "BoltStore" link on README.md

I completed creating [BoltStore](https://github.com/yosssi/boltstore) and added its link on README.md.
Merge pull request #198 from yosssi/patch-1

Add "BoltStore" link on README.md
Update README.md
Merge pull request #199 from kardianos/patch-1

Update README.md