sql: rename sql_vdbe_mem_alloc_region helper

Changed sql_vdbe_mem_alloc_region routine name to the
vdbe_mem_alloc_blob_region because we are going to introduce
a new function with a really similar (but more appropriate) name
vdbemem_alloc_on_region in following patch. Due to the fact that
the function used only in vdbe.c module it has been moved there
and has been marked as "static".

Needed for #2200, #4113, #2233