dial: limit error size

[Stebalien]

Instead of storing every error, store at most 32 errors (plus a "too many errors" error).

Helps address #119

Note: libp2p/go-libp2p-kad-dht#328 actually fixes the issue but this PR helps reduce the impact of similar issues.

[vyzo]

this might even be too many; maybe 16 or 10?

[raulk]

For usability, it would be cool to:

• include the value of maxDialErrors inside the message for ErrTooManyErrors, e.g. "too many errors (max tracked: 32)".
• turn ErrTooManyErrors into a struct that counts how many more errors we found beyond our max; when we reach the threshold, we can retain a pointer to it and keep incrementing the counter for every additional error. That way we could, at least, tell the user how many more errors we saw but failed to retain vs. throwing our hands up in the air entirely.

[Stebalien]

Alternative here: #120.

[Stebalien]

(closing in favor of the alternative, we can reopen if necessary)