Combining relay requests with small value on Home chain

[akolotov]

Currently the users cannot make transfer through the bridge if the value is below the minimum per transaction threshold. From other side, high gas fees force the validators to increase this threshold to make the process of the tokens exchanging less crucial economically. It leads to the situation when the users cannot send transactions through the bridge that transfer small values.

The ability to combine several transaction with small values into one relay request could allow ordinary users to send assets through the bridge without economical impact for the bridge validators.

This functionality is firstly required for the OmniBridge (the multitoken extension for AMB) and for the xDai bridge (erc20-to-native). Consider this issue as the epic and create separate issues for separate bridge components.