sql: refactor sql_atoi64()

We are going to allow using unsigned values in SQL and extend range of
INTEGER type. Hence, we should be able to parse and operate on integers
in range of [2^63, 2^64 - 1]. Current mechanism which involves
sql_atoi64() function doesn't allow this.

Let's refactor this function: firstly, get rid of manual parsing and use
strtoll() and strtoull() functions from standard library. Then, let's
return sign of parsed literal. In case of success now function returns 0,
-1 otherwise.

This patch also inlines sql_dec_or_hex_to_i64() to place of its only
usage: it makes code cleaner and more straightforward.

Needed for #3810
Needed for #4015