Use the standard transaction visibility check for cleaning up cached overflow items.  This fixes an address-sanitizer failure in test/format:

==74450==ERROR: AddressSanitizer: heap-use-after-free on address 0x61d003f3ed4c at pc 0xa123cc bp 0x7fed80381a70 sp 0x7fed80381a68
0x61d003f3ed4c is located 716 bytes inside of 2135-byte region [0x61d003f3ea80,0x61d003f3f2d7)
freed by thread T182 here:
    #0 0x43aee4 in free (/home/mjc/wt/src/wiredtiger/build_posix/test/format/t+0x43aee4)
    #1 0x55aecb in __wt_free_int /home/mjc/wt/src/wiredtiger/build_posix/../src/os_posix/os_alloc.c:234
    #2 0xb31d93 in __ovfl_txnc_wrapup /home/mjc/wt/src/wiredtiger/build_posix/../src/btree/rec_track.c:741