Report cache_peer context in probe and standby pool messages

[eduard-bagdasaryan]

The absence of the usual "current master transaction:..." detail in
certain errors raises "Has Squid lost the current transaction context?"
red flags:

ERROR: Connection to peerXyz failed 

In some cases, Squid may have lost that context, but for cache_peer TCP
probes, Squid has not because those probes are not associated with
master transactions. It is difficult to distinguish the two cases
because no processing context is reported. To address those concerns,
we now report current cache_peer probing context (instead of just not
reporting absent master transaction context):

ERROR: Connection to peerXyz failed
    current cache_peer probe: peerXyzIP

When maintaining a cache_peer standy=N connection pool, Squid has and
now reports both contexts, attributing messages to pool maintenance:

ERROR: Connection to peerXyz failed
    current cache_peer standby pool: peerXyz
    current master transaction: master1234

The new PrecomputedCodeContext class handles both reporting cases and
can be reused whenever the cost of pre-computing detailCodeContext()
output is acceptable.

[rousskov]

Thank you. I expect the new PrecomputedCodeContext class to be reused beyond cache_peer handling code.

We should also report more details for cache_peer connection failures, but those changes deserve a dedicated PR (and will benefit from these context reporting improvements).