Tweak concurrent use numbers

[flub]

There are several numbers that need to be tweaked for the new system:

• We can have a lot of concurrent probes. We should.
• We can reduce the number of concurrently allowed paths.
• The CID count per path can be set back to 5.

[flub]

I've had a look at it seems like currently we issue 5 CIDs per path. So we only need to set MAX_MULTIPATH_PATHS to 8 (from 12) for this issue. I believe the other limits are sensible.

[matheus23]

@flub I opened n0-computer/iroh#4213 to handle more parts of this on the iroh side (after seeing your comment).