Merge the develop branch to the master branch, preparation to v5.0.0

[akolotov]

This update is to finalize work to implement multiple messages support in one transaction made (#403). It contains the following set of improvements and fixes:

• [Improvement] Move STAKE fee distribution implementation into Fee Manager (Hiding of fee distribution implementation #416)
• [Improvement] Add native to erc20 mediator on top of AMB (Add native to erc20 mediator on top of AMB  #367), closes native-to-erc20 AMB extension #364
• [Improvement] Multi AMB requests in one transaction (Multi AMB requests in one transaction #403), closes Several request to AMB in one transaction #307
• [Fix] Remove unneeded checks in Zero DSR tests (Remove redundant asserts in Zero DSR tests #414)
• [Fix] Update block reward mock (Update block reward mock to match implementation #417)
• [Fix] Add method for handling imbalance in SAI tokens (Add method for handling imbalance in SAI tokens #419), closes handle imbalance caused by Sai transferred after ES #418
• [Other] Detailed description of native-to-erc677 extension call flows (Detailed description of native-to-erc677 extension call flows #420)
• [Other] detailed description of STAKE extension call flows (Detailed description of STAKE extension call flows #421)
• [Other] Bump version to 5.0.0 before merging to master (Bump version to 5.0.0 before merging to master #422)