hints: don't read hint files when it's not allowed to send
hints: don't read hint files when it's not allowed to send

When there are hint files to be sent and the target endpoint is DOWN,
end_point_hints_manager works in the following loop:

- It reads the first hint file in the queue,
- For each hint in the file it decides that it won't be sent because the
  target endpoint is DOWN,
- After realizing that there are some unsent hints, it decides to retry
  this operation after sleeping 1 second.

This causes the first segment to be wholly read over and over again,
with 1 second pauses, until the target endpoint becomes UP or leaves the
cluster. This causes unnecessary I/O load in the streaming scheduling
group.

This patch adds a check which prevents end_point_hints_manager from
reading the first hint file at all when it is not allowed to send hints.

First observed in #6964

Tests:
- unit(dev)
- hinted handoff dtests

Closes #7407

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla