Fix bigint marshal, unmarshall

[illia-li]

Fixed for bigint type:

1. string marshal and unmarshal as nullable.
   1.1. Before: marshals "" - caused an error; unmarshals nil data into "0".
   1.2. Now: marshals and unmarshals nil data - "".
2. custom string marshals and unmarshals, before not supported, now supported.
3. big.Int marshals, before not supported, now supported.
4. Marshal data, which equal math.MaxUint64, into uint64, uint, return error' before, now no error`.
5. Unmarshall data with the length is different from 0 and 8, did not return error before, now it returns `error'.

Close issues for bigint:

1. Marshal, unmarshal. For many cql types unmarshaling data that have bigger len than cql type, do not return an error. #246
2. Marshal, unmarshal. For many cql types a go string is supported, but not a go custom string #243
3. Marshal, unmarshal. cql types: tinyint, smallint, int can not be marshaled into go big.Int #244
4. Marshal, unmarshal. Problems with zero and null data and values. #250
5. Marshal, unmarshal. For many cql types unmarshaling data that have smaller len than cql type, do not return an error. #252

[illia-li]

Please take a look again

[sylwiaszunejko]

for me it LGTM, @dkropachev could you take a look?

[dkropachev]

Why did you change it to marshalCounter ?

[illia-li]

I didn't see these changes.
Renamed into marshalBigIntOld.