control/controlclient: flesh out mapSession to break up gigantic method

Now mapSession has a bunch more fields and methods, rather than being
just one massive func with a ton of local variables.

So far there are no major new optimizations, though. It should behave
the same as before.

This has been done with an eye towards testability (so tests can set
all the callback funcs as needed, or not, without a huge Direct client
or long-running HTTP requests), but this change doesn't add new tests
yet. That will follow in the changes which flesh out the NetmapUpdater
interface.

Updates #1909

Change-Id: Iad4e7442d5bbbe2614bd4b1dc4b02e27504898df
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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