lock transactions: avoid redundant reading in write transactions

Our `LockTransaction` class was reading overly aggressively.  In cases
like this:

```
1  with spack.store.db.read_transaction():
2    with spack.store.db.write_transaction():
3      ...
```

The `ReadTransaction` on line 1 would read in the DB, but the
WriteTransaction on line 2 would read in the DB *again*, even though we
had a read lock the whole time.  `WriteTransaction`s were only
considering nested writes to decide when to read, but they didn't know
when we already had a read lock.

- [x] `Lock.acquire_write()` return `False` in cases where we already had
       a read lock.

lib/spack/llnl/util/lock.py

@@ -275,7 +275,12 @@ def acquire_write(self, timeout=None):
             wait_time, nattempts = self._lock(fcntl.LOCK_EX, timeout=timeout)
             self._acquired_debug('WRITE LOCK', wait_time, nattempts)
             self._writes += 1
-            return True
+
+            # return True only if we weren't nested in a read lock.
+            # TODO: we may need to return two values: whether we got
+            # the write lock, and whether this is acquiring a read OR
+            # write lock for the first time. Now it returns the latter.
+            return self._reads == 0
         else:
             self._writes += 1
             return False