Hangs forever in refresh after 5 minutes when 2ary unreachable

tl;dr setsockopt(SO_RCVTIMEO) doesn't work for us in Ruby 2.[01] MRI, infinite hangs result

We're having trouble in our Mongo cluster when a secondary becomes unreachable (due to e.g. crashed instance or network partition): the symptom is that after five minutes all attempts to contact the primary seem to hang indefinitely (or until Unicorn kills our worker process).  Note that our `mongoid.yml` has `options: {read: primary}` 

Attempting to replicate the problem in a test environment gets us a call stack something like this (some frames omitted for brevity)

```
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection/socket/connectable.rb:47:in `read'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection/socket/connectable.rb:47:in `block in read'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection/socket/connectable.rb:119:in `handle_socket_errors'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection/socket/connectable.rb:47:in `read'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection.rb:221:in `with_connection'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/connection.rb:114:in `read'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/protocol/query.rb:213:in `receive_replies'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/node.rb:180:in `block in ensure_connected'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/node.rb:432:in `refresh'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/cluster.rb:182:in `block in refresh'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/cluster.rb:151:in `nodes'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/cluster.rb:240:in `with_primary'
     # /home./.bundler/ruby/2.1.0/moped-4c2a0a29122e/lib/moped/read_preference/primary.rb:55:in `block in with_node'
```
and after some digging the problem seems to be that implementing timeouts using socket options doesn't work in Ruby MRI.  Looking at it with strace I see a call to `read` that returns after five seconds with EAGAIN followed by a call to `ppoll`  - the EAGAIN doesn't ever get as far as interpreted code.

```
getsockname(7, {sa_family=AF_INET, sin_port=htons(44829), sin_addr=inet_addr("192.168.195.6")}, [16]) = 0
setsockopt(7, SOL_SOCKET, SO_SNDTIMEO, "\5\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0
futex(0x7f24117f6654, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0x7f24117f66d0, FUTEX_WAKE_PRIVATE, 1) = 0
select(8, [7], NULL, [7], {0, 0})       = 0 (Timeout)
read(7, 0x7f2413130f00, 100)            = -1 EAGAIN (Resource temporarily unavailable)
ppoll([{fd=7, events=POLLIN}], 1, NULL, NULL, 8  <hang indefinitely>
```

See also http://stackoverflow.com/questions/9853516/set-socket-timeout-in-ruby-via-so-rcvtimeo-socket-option

The attached patch has a spec that demonstrates the problem (try running it under strace) and a fix which implements read timeouts "by hand" using select() instead of relying on socket options.  A more full solution would probably address the write() case and be tested on SSL connections as well as plaintext ones, but I thought I'd push this early diagnosis up to you for comment first to see if I'm on the right track

/cc @dawid-sklodowski @cpoo22 just fyi