libostree: Add multiple getter/setter support to OstreeAsyncProgress

libostree: Add multiple getter/setter support to OstreeAsyncProgress

OstreeAsyncProgress is thread-safe: it can have keys changed by one
thread while another is getting the same keys (modulo some locking
contention). However, the thread safety is done at the function call
level: if some code calls an OstreeAsyncProgress getter several times,
the key fetches are not atomic with respect to each other.

In the case of contention on the lock, this can result in consumers of
OstreeAsyncProgress data seeing an inconsistent state between the
properties they query, which could result in progress reporting
inaccuracies.

In the uncontested case, this results in the OstreeAsyncProgress lock
being locked and unlocked many times more than necessary.

Try to improve this by adding new API, which supports getting and
setting multiple keys atomically:
 • ostree_async_progress_get()
 • ostree_async_progress_set()

The new API uses GVariants and varargs: keys are passed as a
GVariantType string followed by arguments as for g_variant_new() or
g_variant_get(), followed by the next key, etc.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #819
Approved by: cgwalters