tor-hsservice: Do not store the subcredentials in RendRequestContext.

Previously, the subcredentials were computed in IptEstablisher::launch and stored in RendRequestContext. This caused long-running services to report errors like:

WARN tor_hsservice::helpers: Problem while accepting rendezvous request: error: Could not process INTRODUCE request: Introduction handshake was invalid: Circuit-extension handshake authentication failed

for clients using newer subcredentials than the ones the service had at the time the IPT was established.

Fixes #1242 (closed)

assigned to @gabi-250

requested review from @ahf

@gabi-250 if you want review of this one today, it's probably a good idea to assign it to someone else since I am in 1:1's all days - otherwise I'll look at it tomorrow when I get in.

This one doesn't necessarily need to go in today

This looks pretty reasonable to me!

merged

mentioned in commit 455d0106