General improvements (memory, bot client, containers)

[canercidam]

• Updated forta-core-go to take @aomerk's memory usage improvements in Reduce marshal/unmarshal usage for proto conversions forta-core-go#239
• Removing the unsafe.Pointer hack in gRPC message encoding: It seems to do the inverse of what we expected. The memory usage is lower and less aggressive without it.
• Reusing top-level bot context to implement the "closed" state for the bot and making the input processing loops mind the cancellation of this context
• Removing container images only when unused bots are cleaned up and some bots are removed (not during node exit as the images will be reused after node restart)