utils: fix unbounded chunked_kv_cache index growth

The index was reclaimed by walking a single fifo of evicted entries,
stopping at the first entry still on the ghost queue. Main-queue
evictees never enter the ghost queue and were interleaved behind
ghosted entries, and the ghost clock only advances on small-queue
evictions, so under a touch-heavy workload with never-repeating keys
(e.g. the fetch read coalescer) gc stalled behind one live ghost entry
and the index grew by one entry per insertion.

Erase main-queue evictees from the index at eviction time. This leaves
the ghost fifo holding only ghost-queue entries, whose expiry is
monotone in fifo order, making the gc early-return correct.