crypto: fix another over-run in bio

When doing `FreeEmpty`, `NodeBIO` skips pre-allocated `head_` buffer.
However this might lead to double-freeing buffers since in `~NodeBIO()`
we're starting deallocation from `head_` buffer.
src: use PersistentToLocal() in a few more places

Update a few more `Local<T>::New(isolate, persistent)` call sites to
`PersistentToLocal(isolate, persistent)` - the latter has a fast path
for non-weak persistent references.
src: use static_cast for void-to-T casts

Code cleanup, replace a few uses of reinterpret_cast<T*>(void_ptr) with
static_cast<T*>(void_ptr).
tls: fix handling of `SNICallback` server option

It shouldn't ignore it!

There're two possibile cases, which should be handled properly:

1. Having a default `SNICallback` which is using contexts, added with
  `server.addContext(...)` routine
2. Having a custom `SNICallback`.

In first case we may want to opt-out setting `.onsniselect` method (and
thus save some CPU time), if there're no contexts added. But, if custom
`SNICallback` is used, `.onsniselect` should always be set, because
server contexts don't affect it.
v8: fix openbsd build

This is [1] applied ahead of time. Summary:

    OpenBSD doesn't have <ucontext.h>. ucontext_t lives in <signal.h>
    and is a typedef for struct sigcontext. There is no uc_mcontext.

[1] https://codereview.chromium.org/21705003/
http: add agent.maxFreeSockets option
doc: http keepalive, agent options