memtx: round up memtx_memory to the lower bound if less is set

The memtx engine has two quota consumers: `index_slab_cache` and
`slab_cache`. They both acquire SLAB_SIZE of space on first tuple
insert in "small" allocator mode, so the quota less than the slab
size times two exceeds right there.

It was decided to introduce a theoretically minimal size of the
quota and round-up any value given in box.cfg if it's less than
the lower bound.

Closes #7389

NO_DOC=bugfix

(cherry picked from commit de0731bd)