While looking for the source of a memory leak, I decided to optimize
the metatracer to avoid encoding values when there weren't any tracers
configured to received the encoded values.
I also avoided putting the EVM into debug mode when there weren't any
tracers configured.
None of these fixed the memory leak, but they still seem like good
practices. Memory leak fix is in the next commit.
This makes several updates to support the blockupdates plugin.
I had to update several hooks that were using the wrong types, and
provide a way to get event.Feed objects into plugins without importing
event.Feed (which I did by having the plugin loader make them
available).
When the plugin loader itself had to know the types in the arguments
and return values of the plugin functions, it was very difficult to
avoid import loops, given that the types were often defined in the
same package that needed to invoke the plugins.
Under this model, the plugin loader has much less knowledge of the
plugins themselves, and within each package we define functions to
interact with the plugins.
