[bug] Race condition when excute call on go routine

[himitsuko]

Description

When define an instance of the call and reuse the contract instance to AddCall or Call in more than 1 routines. The race condition was happen.

Reproduce

abi = call.NewContracBuilder().AddMethod(...)

go func(){
  abi.AddCall(...)
  abi.AddCall(...)
  abi.Call(nil)
}()
go func(){
  abi.AddCall(...)
  abi.AddCall(...)
  abi.Call(nil)
}()

Expected

Optimize it for the threadsafe