* Allow the streaming lib resend frequency to drop down to 20s as the
minimum, so that up to 2 retries can get sent on an http request.
* Add further limits to failsafe tunnels.
* Keep exploratory and client tunnel testing and building stats separate.
* Only use the 60s period for throttling tunnel requests due to transient
network overload.
* Rebuild tunnels earlier (1-3m before expiration, by default)
* Cache the next hop's routerInfo for participating tunnels so that the
tunnel participation doesn't depend on the netDb.
* Fixed a long standing bug in the streaming lib where we wouldn't always
unchoke messages when the window size grows.
* Make sure the window size never reaches 0 (duh)
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