sql: rename REAL/FLOAT/DOUBLE types to NUMBER

Before this patch it was allowed to specify REAL/FLOAT/DOUBLE types
which matched with NUMBER NoSQL type in space format. However, NUMBER is
different from standard floating point types, since it is able to hold
integers in range [-2^63; 2^64-1] alongside with double precision
floating point values. Hence, to not confuse users it has been decided
to remove support of REAL/FLOAT/DOUBLE types from SQL grammar and use
instead original NUMBER type naming.