fix: synchronize root cache initialization

[loglapa]

Overview

• synchronize lazy root cache initialization so concurrent Root calls do not race through the shared hasher or cache
• keep a read-locked fast path for an already cached root
• strengthen the existing concurrent regression with a simultaneous start barrier and result equality checks
• document that concurrent Root calls are supported after tree mutation is complete

Closes #338

Verification

• GOCACHE=/private/tmp/nmt-go-build GOMODCACHE=/private/tmp/nmt-go-mod go test -race ./...
• GOCACHE=/private/tmp/nmt-go-build GOMODCACHE=/private/tmp/nmt-go-mod go vet ./...
• git diff --check