Update python APIs to make init return a RecordingHandle which manages most state

[jleibs]

The idea is that the RecordingHandle would be locked to a single recording-id and as long as it's held can continue to be used for logging without some other context (i.e. another jupyter notebook cell) clobbering the sink in the global rerun context)

The basic idea is to enable usage along the lines of the following:

rec1 = rr.init(sink=Tcp)
rec2 = rr.init(sink=File('my.rrd'), make_default=False)
rec3 = rr.init(sink=Memory, make_default=False)

# Different ways to log to a specific recording

rr.log_points(... , recording=rec1)

rec2.log_points(...)

with rec1:  # temporarily pushes the new recording onto a stack for this thread
  rr.log_points(...)

# Or the global default can still be updated, which is the sink used 
rr.set_default_recording(rec3)

[emilk]

Related to

• SDK-side log batching #1880