add abigen command based on ethereum-go v1.9.11

[elv-gilles]

Closes content-fabric#1617

Take abigen from ethereum-go/v1.9.11 and modify the go template to:

• pre-parse ABIs and have the parsed ABIs in a generated map keyed by ABI names
• remove some generated code that is never used
• generate EventInfo structs for 'Unique events'. Unique events are events whose ID and name are unique across contracts. This covers all our events except 'LogPayment' which has 2 versions (the one in content.sol is not used)

  // EventInfo gather information about a 'unique event'.
  type EventInfo struct {
  Name   string                                    // name of the event as in abi.Event
  ID     common.Hash                               // ID of the event
  Type   reflect.Type                              // type of the struct event
  Unpack func(log types.Log, ev interface{}) error // unpack the given log into the given event
  }

• generate maps of unique events:

  
  // Map of Unique events names to *EventInfo.
  // Unique events names are constants starting with E_
  var UniqueEvents = map[string]*EventInfo{}

// Map of Unique events types to EventInfo var EventsByType = map[reflect.Type]EventInfo{}

// Map of Unique events IDs to EventInfo var EventsByID = map[common.Hash]EventInfo{}

The generated `base_content_space.go` has been manually merged with the existing one to preserve the generated byte code (xxBin strings).

--- 
Note: branch `20200805` has similar changes for `133cb66c6e67fc946c6b73d7f7db3bbb913d4859` which is the revision currently used in fabric.

The new go bindings in branch `20200805` have been manually merged with the existing ones in order to preserve the `xxBin` constants (the contract byte code) since the `solc` currently in elv-toolchain produces byte code not exactly the same than the one that was generated at time.

For older versions (like in build/20200206) we could just rewrite the bindings to get the pre-parsed ABIS.