Metadata: add env fields to ease lttng path hierarchy creation for viewer

Metadata: add env fields to ease lttng path hierarchy creation for viewer

Add the following fields in the env section of the metadata:

  - trace_name
      The session name without datetime information.
      Hence when the session is an auto-generated one, only print
      LTTNG_DEFAULT_NAME.
  - trace_creation_datetime:
      The datetime at which the session was created.
      We use session->creation time for it.
  - tracer_buffering_scheme
      The buffering scheme used. The value can be uid or pid.
  - tracer_buffering_id
      The key used by the buffering scheme (uid/pid).
  - architecture_bit_width
      The bit width of the computer architecture (e.g 32 or 64)
  - vpid_datetime
      The registration time of the vpid for per-pid mode.

Adding these fields ensure that the trace itself carry information that
is normally carried via folder hierarchy. e.g

test-20190417-174951/                      <- trace_name, trace_creation_datetime
└── ust                                    <- domain
    └── uid                                <- tracer_buffering_scheme
        └── 1000                           <- tracer_buffering_id
            └── 64-bit                     <- architecture_bit_width
                ├── channel0_0
                ├── index
                │   ├── channel0_0.idx
                └── metadata

Per-pid buffering is quite similar.

auto-20190722-174816                        <- trace_name, trace_creation_datetime
└── ust                                     <- domain
    └── pid                                 <- tracer_buffering_scheme
        └── sample-ust-7640-20190722-174818 <- procname, tracer_buffering_id, vpid_datetime
            ├── my-channel_0
            ├── index
            │   ├── my-channel_0.idx
            ├── metadata

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>