Merge the develop branch to the master branch, preparation to v2.1.0

[akolotov]

This merge contains the following set of changes:

• [Improvement] Add Burner Wallet Plugin for WETC Bridge (Add Burner Wallet Plugin for WETC Bridge #306)
• [Improvement] Cache monitor RPC calls to increase performance (Cache monitor RPC calls #322)
• [Fix] Update monitor half duplex transfer filter (Update monitor half duplex transfer filter #311), closes Monitor may not display the correct information about SAI transfers  #308
• [Fix] Fix monitor balances displayed for erc to native (Fix monitor balances displayed for erc to native #312), closes Fix e2e tests related to disabling SAI transfers #309
• [Fix] Update contract's submodule and e2e tests (Update contract's submodule and e2e tests #313)
• [Other] Update the contract's submodule to the release 5.0.0-rc0 (Update the contract's submodule to the release 5.0.0-rc0 #321)
• [Other] Bump acorn from 5.7.3 to 5.7.4 (Bump acorn from 5.7.3 to 5.7.4 #305)