Fix various possible stalls of read head (i.e. try moving it after every
write head update).

NOTE: This is actually backported from `bud`.

Try writing multiple chunks from NodeBIO if possible.

Otherwise it might get stall (`Peek()` will return zero-length chunk)
in following situation:

1. `Write(kBufferLength)`
2. `Read(kBufferLength)`
3. `Write(anything)`
4. `Peek()` => `len=0`

Remove NodeBIO::GetMethod() and replace calls to BIO_new() with calls
to the new NodeBIO::New() function.

This commit basically reshuffles some code in order to make it explicit
that the NodeBIO BIO_METHOD is const.

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.

Might cause write head running over read head, when there were no
allocation and `Commit()` was called. Source of at least one test
failure on windows (`simple/test-https-drain.js`).

Read head can be the same as write head, even if there's some data to
read.