Update agent pprof rpcs to streaming

### Issue

Improve upon #6841 to use streaming rpc endpoints so that the entire profile isn't loaded into memory. On a buys client or server a long run trace profile can reach near 100mb. Streaming would alleviate some of the memory burden on the server instead of loading it entirely in memory.

See https://github.com/hashicorp/nomad/pull/6841#issuecomment-572731049 for memory characteristics of various profiles.

Start with the agent pprof RPCs and the prior work in issue #6841, including its linked discussion about profile memory characteristics. The work is complete when long-running trace profiles use streaming RPC endpoints rather than loading the entire profile into memory.

Written by the indexing model from the issue text.