WIP fine-grained task concurrency

[lihaoyi]

This code is intended to remove the global out/ folder lock in favor of finer grained per-task or per-bootstrap-phase read/write locks, with handling for the various "global" out/ folder files (e.g. mill-chrome-profile.json,
mill-profile.json, mill-console-tail, etc.) to preserve their usefulness in the presence of concurrent runs.

• During task execution, we take a read lock for each task, if it needs to be re-evaluated we then take a write lock, and when evaluation is done we keep the read lock to ensure it remains stable for use by downstream tasks until execution completes

• When evaluating meta-builds, we take a write lock for any meta build that is being actively evaluated, and convert to a read lock after it is done so it remains stable while we use it

• The various out/mill-* files are turned into symlinks to out/mill-run-*/* files: the out/mill-* file pointing at the most recent out/mill-run-*/* file to be created, but the older out/mill-run-*/* files kept around in case people need them (e.g. due to concurrent use)

• RunnerState handling is updated to support concurrent runs, with the same workerCache shared across all runs

• This change is meant to only affect daemon mode, since we don't have reliable filesystem locking we can rely on for non-daemon mode (e.g. docker folder mounts don't always support locks properly). mill-out-lock is turned into a process-level lock, such that either a single MillDaemonMain process or MillNoDaemonMain process can claim it. If a MillDaemonMain claims it then it can support multiple concurrent runs with fine-grained locking internally, but if a MillNoDaemonMain claims it then only that one run can take effect