perf map fixes

[alban]

• vendor fixes from gobpf
• tcptracer-bpf: change the size of the ring buffer "maps/tcp_event_ipv4" to 256 pages (1MB)
• tcptracer-bpf: use the go channel to notify lost events

Do we need two different callbacks for IPv4 and IPv6? With the current code, the lostCb could be called from two different go routines in parallel.

Is 1MB a reasonable size for a ring buffer? On my laptop with 4 cpus, that would mean 4MB in total.

Depends on

• elf: perf ordering fixes iovisor/gobpf#43
• perf maps: add configurable parameters iovisor/gobpf#44
• update build-tools git-subtree #38
• update build-tools git-subtree (again) #40

Useful for weaveworks/scope#2507

[alban]

The build failure comes from build-tools. The fix is available in:
weaveworks/build-tools@e506f1b
I'll update the git-subtree.

[alban]

I re-vendored the last gobpf. shfmt issues should be fixed too.
@2opremio PTAL