fix(connext): remove BigInt to avoid precision loss

[ghost]

This removes BigInt conversion from ConnextClient since it was causing precision loss when sending requests to Connext API.

EDIT by @kilrau : fixes #1887

[sangaman]

Do we know why BigInt was causing loss of precision? I couldn't find anything about it and logically it seems like it should work as it was. However, since units is already a number type, it's not like we can gain back any precision by converting to BigInt either, so the new approach looks like an improvement. If we still have issues with this maybe we need to revisit the amount/unit conversion logic and consider using other types than the native JS number.

[ghost]

Do we know why BigInt was causing loss of precision?

I'm not 100% sure, but the steps conversion looked like Number -> BigInt -> String VS Number -> String in this PR. Guess it ties back to how floating point numbers in JavaScript work.

If we still have issues with this maybe we need to revisit the amount/unit conversion logic and consider using other types than the native JS number.

I'm pretty convinced there are still edge cases that our current implementation does not cover. Ideally, I'd like to see all quantities in/out of xud gRPC layer in string format and removing the amount/unit (satoshis per coin) from internal xud logic.