sql: make mem_is_bin() to check only for VARBINARY

After this patch, the mem_is_bin() function will return 'true' only if
the value that the MEM contains is of type VARBINARY. This patch also
adds the mem_is_bin_ext() function, which is used to check if a MEM
contains value of type VARBINARY or value of types that are currently
considered VARBINARY extensions - MAP and ARRAY.

Part of #4906