Partial bug #2964 fix: Avoid stuck ICAP REQMOD transactions when HTTP fails.

Partial bug #2964 fix: Avoid stuck ICAP REQMOD transactions when HTTP fails.

Added doneWithRetries() and used it to inform the request body sender that
there will be no more body consumers. This allows the sender (e.g., an ICAP
REQMOD transaction) to quit instead of waiting for body buffer space forever.

Moved !self check into checkRetry() because we need to call doneWithRetries()
even if self is nil. The move should not affect the old code.

Based on lp 3p1-rock branch, r9610.