Fix a leak in eviction detected by valgrind.

832,394 (518,080 direct, 314,314 indirect) bytes in 32,380 blocks are definitely lost in loss record 61 of 61
    at 0x4A06B6F: calloc (vg_replace_malloc.c:593)
    by 0x413A0D: __wt_calloc (os_alloc.c:38)
    by 0x436131: __wt_rec_evict (rec_evict.c:207)
    by 0x4235DE: __evict_page (bt_evict.c:511)
    by 0x42428E: __wt_evict_lru_page (bt_evict.c:1235)
    by 0x42506A: __wt_cache_evict_server (bt_evict.c:870)
    by 0x3E39407D14: start_thread (pthread_create.c:308)
    by 0x3E38CF246C: clone (clone.S:114)
Stop trying to cache the oldest reader: we only use it for eviction and only update it when required.