Regularly dump TCP_INFO statistics of outgoing connections

Backport from PR #401 to 1.7.

(cherry picked from commit b88bd91447cb143ae5a6a08ca54a9d5166314d6e)

Conflicts:
	src/libcutil.clib

Closes #401
Rework TLS configuration
Add new TLS configuration options to sample file
Read & apply configured TLS version & cipher list
Add TLS protocol configuration support to client

This commit also adds a `-tls-version` argument to the CLI interface in
order to select a TLS protocol version when using it.
Add note to Python client about TLS versions

Python 2.x doesn't do anything higher than TLSv1...
CI: Require `ssl-0.4.7` & update seed image
Doc: Add new TLS settings to documentation
Merge pull request #456 from Incubaid/tcpinfo-for-1.7

Regularly dump TCP_INFO statistics of outgoing connections
Enhance TLS key and cert configuration checking

Result of the review in PR #461.

See: https://github.com/Incubaid/arakoon/pull/461#discussion-diff-14136674
Merge pull request #461 from Incubaid/tls-settings

Add TLS version & cipher list configuration
Merge remote-tracking branch 'incubaid/1.7' into 1.6_for_1.7_bis

Conflicts:
	src/msg/tcp_messaging.ml