box: check fiber slice in generic implementation of index count

`index.count()` may hang for too long in Vinyl if a substantial
consecutive hunk of the space is stored in memory. Let's add
a fiber slice check to it to prevent it from blocking the TX thread
for too long.

Closes #10553

NO_DOC=bug fix

(cherry picked from commit e19bca5a74e83d2521fe770f2a93c3e3d3ad4801)