build: fix broken boost chrono check on some platforms

If clock_gettime is implemented outside of libc (librt in this case), configure
would fail when testing boost. Since clock_gettime is not present on all OSs,
boost only uses it when it can. Check for it in librt and add it to LIBS if
found, but don't fail if it's not (since boost won't be expecting it in this
case).

Also, reverse the link order as necessary for static libs.

Note that it's possible that there are other similar cases for boost, which may
be handled the same way.

Rebased-from: 460b32d
gitian: remove unneeded option after last commit

Rebased-from: a7ec027
Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).

Rebased-from: f1920e8
Add nLocalServices info to RPC getinfo

Also show full 64 bits of services. Previously service bits >32 that
were advertised just didn't show up at all.

Rebased-from: 99ddc6c
Add comment regarding experimental-use service bits

As per mailing list discussion.

Rebased-from: 38405ac