The `README.md` links the Gitter channel for discussions, but the
official docs and even the Gitter channel itself recommend using the
official Discord Server for such discussions.
This PR simply changes the Gitter link and provides Discord invite link.

travis: enable experimental docker for manifest building

travis, Dockerfile, build: docker build and multi-arch publish combo

travis: move docker steps further to prevent hanging other builders

When deleting in fullNode, and the new child node nn is not nil, there is no need
to check the number of non-nil entries in the node. This is because the fullNode 
must've contained at least two children before deletion, so there must be another
child node other than nn.
Co-authored-by: Felix Lange <fjl@twurst.com>

Copy the CliqueConfig instead of reusing the pointer.
This makes DeveloperGenesisBlock thread safe and prevents it from
changing params.AllCliqueProtocolChanges.Clique.Epoch.

travis, build: add support for multi-arch docker images

travis: don't overwrite amd64 images with arm64

travis, build: own docker builder and hub pusher

* core/vm: evm fix panic

* core/vm/runtime: default to params.initialbasefee

core, graphql, internal: expose effectiveGasPrice in receipts

params: bump CHTs for Geth v1.10.4

* accounts/abi/bind: fix bounded contracts and sim backend for 1559

* accounts/abi/bind, ethclient: don't rely on chain config for gas prices

* all: enable London for all internal tests

* les: get receipt type info in les tests

* les: fix weird test
Co-authored-by: Martin Holst Swende <martin@swende.se>

* params: add london testnet blocks

* core/forkid: update fork hashes

core: change baseFee into baseFeePerGas in genesis json

* accounts/abi/bind/backends: add blockByHashNoLock
Signed-off-by: Oliver Tale-Yazdi <oliver@perun.network>

* accounts/abi/bind/backends: add 'parent' arg to rollback
Signed-off-by: Oliver Tale-Yazdi <oliver@perun.network>

* accounts/abi/bind/backends: add simulated forks
Signed-off-by: Oliver Tale-Yazdi <oliver@perun.network>

* accounts/abi/bind/backends: minor nitpicks

* accounts/abi/bind/backends: don't add defensive panics
Co-authored-by: Péter Szilágyi <peterke@gmail.com>

core: make genesis parse baseFee correctly

core, internal: support various eth_call invocations post 1559

This function is not used in the code base, so probably safe to do rename, or remove in its entirety, but I'm assuming the logic from the original creator still applies so rename probably better.

eth/ethconfig: bump the RPC gas cap to 50M, since 1559 exceeds 25