add checksum offset to avoid bigint overflow

[vvkh]

We utilize a portion of the MD5 checksum, specifically 6 bytes per row, to allow for the accumulation of more rows before encountering overflow. However, in most databases, hex numbers are padded with zeros if they are not 8 bytes long. For example, 0xFF becomes 0x00000000000000FF.

Since the most significant bit is always 0, the number is consistently positive. Consequently, there is a limit to the number of values we can sum before encountering overflow in the bigint type.

To address this limitation, we introduce a negative offset equal to half of the maximum checksum number. This adjustment expands the range of values to "-max number/2" to "+max number/2". By incorporating negative numbers, we can offset some of the positive ones, enabling the summation of more numbers without encountering bigint overflow.

Smoke tests for some databases after update
Compute checksum from a string literal "hello"

db	expr	checksum
clickhouse	reinterpretAsUInt128(reverse(unhex(lowerUTF8(substr(hex(MD5('hello')), 21))))) - 140737488355327	32508877456787
bigquery	cast(cast(('0x' || substr(TO_HEX(md5("hello")), 21)) as int64) as numeric) - 140737488355327	32508877456787
snowflake	BITAND(md5_number_lower64('hello'), 281474976710655) - 140737488355327	32508877456787
postgres	('x' || substring(md5('hello'), 21))::bit(48)::bigint - 140737488355327	32508877456787
mssql	convert(bigint, convert(varbinary, '0x' + RIGHT(CONVERT(NVARCHAR(32), HashBytes('MD5', 'hello'), 2), 12), 1)) - 140737488355327	32508877456787
mysql	conv(substring(md5('hello'), 21), 16, 10) - 140737488355327	32508877456787
oracle	to_number(substr(standard_hash('hello', 'MD5'), 21), 'xxxxxxxxxxxxxxx') - 140737488355327	32508877456787