Accounting packet lost and discarded events at runtime

This patchset is composed of commits in lttng-tools, and lttng-ust, it allows the sessiond, to report to the user the
discarded events (when a channel is in discard mode) or the lost packets (when a channel is in overwrite mode).

Please pull from:
https://github.com/jdesfossez/lttng-tools-dev/tree/seqnum
a65a791 Store the instance id and packet_seq_num in indexes 8049650 kernctl commands to extract the stream instance id f947d62 Account the lost packets in snapshot mode 978bee2 Display discarded and lost events at destroy and stop 77f57b2 Extract the lost packets and discarded events counters 133e6c4 Metadata for instance_id and packet_seq_num 3121370 Add a HT to lookup sessions by id
bda9cd8 Explicitly stop the session on lttng destroy

https://github.com/jdesfossez/lttng-ust-dev/tree/seqnum
e25f0ef Export the stream instance ID f01b21f Generate and export the sequence number 593dff9 Add stream instance id to the packet header

Changes from v3:
- lttng-modules patches Acked-by and merged in master
- lttng-ust patches all Acked-by
- removed the back reference introduced to retrieve the lttng_ust_session and channels from the ust_app_chan
- introduced a new hashtable to find easily ltt_session from their id.

Changes from v2:
- actually pushed the updates intended for v1 in UST and tools...
- fixed the comments from Mathieu on the v2

Change from v1:
- the packet sequence number is not dependant on the subbuf_size and now always a uint64_t, so it got rid of the need of the packet_seq_num_padding field

Original cover letter:
The reporting is detailled on a per-channel basis in the "lttng list" command (and associated API calls) at runtime. On lttng stop/destroy, a warning is now displayed to the user if events/packets were discarded/lost during the session.

The information about the packet lost is extracted from a packet sequence number now written in each CTF packet and CTF index, allowing the relayd and the viewers to know when parsing the trace/index that entire packets were lost between two data packets.

Finally, a stream_instance_id is now written in each packet, this feature allows a viewer to link multiple tracefiles belonging to the same channel and stream. This is particularly useful when dealing with tracefile rotation because the viewers will now be able to compute the accurate number of discarded events.

Julien

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