Manage handlers with shared_ptrs, introduce StaticPipeline

Manage handlers with shared_ptrs, introduce StaticPipeline

Summary:
A few things:
- Eliminate HandlerPtr by managing all handlers with shared_ptrs instead of keeping them inline in the pipeline
- Kill recursively templated ChannelPipeline accordingly
- Introduce StaticPipeline to retain the flexibility of zero-alloc pipelines
- Introduce notion of an "owning handler" to avoid destruction order issues

Test Plan: unit (will add more), thrift unit

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, alandau, bmatheny, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2023976

Tasks: 6836580

Signature: t1:2023976:1430159578:e50e8a149e549a40670d093fb65987a4843cdd8d