Clarify HostDBInfo state

[masaori335]

Summary

Replaces the "alive/dead/ombie" terminology in HostDBInfo with an explicit three-state model, "up/down/suspect", and cleans up the API surface.

HostDBInfo State

State	Description
Up	No known failure; eligible for normal selection.
Down	Blocked; no connections permitted until _last_failure + fail_window elapses.
Suspect	Fail window has elapsed; connections are permitted. On success transitions to Up; on failure returns to Down.

Changes

HostDBInfo class

• Converts struct to class with private _last_failure / _fail_count fields
• Adds State enum (UP, DOWN, SUSPECT) and a canonical state(now, fail_window) method
• Sugar helpers is_up(), is_down(now, fail_window), is_suspect(now, fail_window) for call-site clarity
• mark_down(now, fail_window) now handles both UP→DOWN and SUSPECT→DOWN transitions; the latter refreshes the fail window by updating _last_failure
• increment_fail_count takes fail_window so it can delegate to the updated mark_down
• Moves all method implementations out of the header and into HostDBInfo.cc

Documentation

• Replaces "zombie" with "suspect" throughout hostdb.en.rst

Tests

• New test_HostDBInfo.cc with unit tests covering all state transitions including concurrent CAS races and full UP→DOWN→SUSPECT→DOWN→SUSPECT→UP cycles