Stores: lock(key), an exclusive named lock (#70)

[thorwhalen]

Adds store.lock(key): an exclusive, named context manager scoped to the store's data (for PostgresStore, its owner and project). Needed by reeleehq/reelee#632.

• IntervalAnnotationStore.lock(key) in the protocol, with the semantics documented there: same key excludes, different keys do not, a dead holder releases it, not re-entrant.
• PostgresStore: session-level pg_advisory_lock keyed on a 64-bit blake2b hash of owner + project + key (length-prefixed parts, so no concatenation collisions), on a dedicated connection and released in finally; a crashed holder frees it when its connection drops.
• SqliteStore: a file lock (flock, msvcrt on Windows) on a sidecar file per key beside the .annot; excludes threads and other processes on the host. :memory: falls back to a process-local lock.
• MemoryStore / MappingStore: process-local named locks; copies and pickles get fresh locks.
• Mechanisms live in the new lacing/store/locking.py.

Tests: tests/test_store_lock.py (all in-process backends, plus a cross-process SQLite case) and TestLock in tests/test_store_postgres.py, run against a real PostgreSQL server: two connections contend for one key (second blocks until the first releases), different keys and different projects do not contend, release on exception. Full suite: 906 passed.

Closes #70

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