sql: fix passing FP values to integer iterator

Before this patch it was impossible to compare indexed field of integer
type and floating point value. For instance:

CREATE TABLE t1(id INT PRIMARY KEY, a INT UNIQUE);
INSERT INTO t1 VALUES (1, 1);
SELECT * FROM t1 WHERE a = 1.5;
---
- error: 'Failed to execute SQL statement: Supplied key type of part 0 does not match
    index part type: expected integer'
...

That happened due to the fact that type casting mechanism (OP_ApplyType)
doesn't affect FP value when it is converted to integer. Hence, FP value
was passed to the iterator over integer field which resulted in error.
Meanwhile, comparison of integer and FP values is legal in SQL.  To cope
with this problem for each equality comparison involving integer field
we emit OP_MustBeInt, which checks whether value to be compared is
integer or not. If the latter, we assume that result of comparison is
always false and continue processing query.  For inequality constraints
we pass auxiliary flag to OP_Seek** opcodes to notify it that one of key
fields must be truncated to integer (in case of FP value) alongside with
changing iterator's type: a > 1.5 -> a >= 2.

Closes #4187