Metadata: add env fields to ease life of viewer

Metadata: add env fields to ease life of viewer

Add the following field 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_time:
      The time 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).
  - isa_length
      The length of the ISA used. (e.g 32 or 64)

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

test-20190417-174951/                      <- trace_name
└── ust                                    <- domain
    └── uid                                <- tracer_buffering_scheme
        └── 1000                           <- tracer_buffering_id
            └── 64-bit                     <- isa_length
                ├── channel0_0
                ├── channel0_1
                ├── channel0_2
                ├── channel0_3
                ├── index
                │   ├── channel0_0.idx
                │   ├── channel0_1.idx
                │   ├── channel0_2.idx
                │   └── channel0_3.idx
                └── metadata

Per-pid buffering is quite similar.

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