Improve transaction pool

[ninjaahhh]

two main features:

1. better sorting (gas price, and may also take multi native token into account)
2. support pending tx with higher nonce, instead of throwing them away

for you to triage @qizhou

[ninjaahhh]

for dealing with tx with really low gas fee, should we 1) enforce a minimum gas price, or 2) simply enforce a limit to the size of tx queue or add a "self-cleaning" method to discard low-fee tx if reaching a threshold?

update: tracked here #574

[ninjaahhh]

for sorting by multi-native token gas price, a separate issue is created as #577 which is a stretch goal