crypto: make error buffer non-static

Doesn't matter now but it will if/when we have support for multiple
threads.
crypto: remove misleading comment
crypto: make root_cert_store variable extern

Before this commit it was declared static (in a header file!), meaning
it got duplicated in every file that includes it.

A few duplicated pointers is not the end of the world but it introduces
a lot of potential for confusion because root_cert_store in file A is
not the root_cert_store in file B.

Moral of the story: don't declare static variables in header files.
crypto: remove NodeBIO::GetMethod()

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.
src: remove unused Persistent<FunctionTemplate>