feature/featuretags: add LazyWG modular feature

Due to iOS memory limitations in 2020 (see
https://tailscale.com/blog/go-linker, etc) and wireguard-go using
multiple goroutines per peer, commit 16a9cfe2f4ce7d introduced some
convoluted pathsways through Tailscale to look at packets before
they're delivered to wireguard-go and lazily reconfigure wireguard on
the fly before delivering a packet, only telling wireguard about peers
that are active.

We eventually want to remove that code and integrate wireguard-go's
configuration with Tailscale's existing netmap tracking.

To make it easier to find that code later, this makes it modular. It
saves 12 KB (of disk) to turn it off (at the expense of lots of RAM),
but that's not really the point. The point is rather making it obvious
(via the new constants) where this code even is.

Updates #12614

Change-Id: I113b040f3e35f7d861c457eaa710d35f47cee1cb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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