Get rid of DBM, which technically is a case of YAGNI. Supernodes will need a way to save identities, but that can be a different feature. Regular clients do not really need a permanent cache (yet). When/if we do need one we can do it then. Until then it only caused problems.
Peers are now dumped on shutdown in a persistence cache and reloaded on startup, which is good enough for clients right now. Supernodes will get something else for long-term authoritative identity caching.
Add persistent identity caching for use on supernodes. Activate by just making an iddb.d directory in the ZeroTier home folder. Also clean up some obsolete cruft from makefiles.
Version 0.6.1: minor bug fix, DBM removal

This version removes the peer DBM present in earlier releases. It is not necessary for
regular clients and has been a source of problems.

There is a long-term identity cache that can be enabled by making a directory called
"iddb.d" in the home folder and restarting ZT1. This is probably something only our
supernodes would need, since regular nodes can easily WHOIS peers they've forgotten
about.

On shutdown, the peer database is dumped to disk. It's then restored on startup.
Peers that have not been used in a while are cleaned out, so this keeps this data
set small.

A DBM may re-appear later if it's needed, but for now it was YAGNI.