Generalize ef_extend() to non-file-backed tables

Generalize ef_extend() to non-file-backed tables

ef_extend() extended the file bypassing the cache, which screws up the
cache if it's EFF_MEM.  It fixed that by closing and reopening the
table.  Cheesy, and worked only for file-backed tables.

Rewrite ef_extend() to remap the cache properly for EFF_MEM.  While
there, simplify the !EFF_MEM case: steal a cache slot instead of
allocating a buffer.

Factor cache mapping out of ef_open() and ef_extend() into new
ef_realloc_cache().