We already have 3 places where we create picklers and unpicklers:
zodb/pydata.go, fs1/index.go and zeo/proto.go and they are already
diverging a bit: for example pydata was explicitly specifying protocol
for pickle encoder, while other places were using defaults.

We will soon want to use StrictUnicode option for all picklers and
unpicklers we create, which means it is better to keep this
customization only in one place instead of copy-pasting it in between
callsites increasing the possibility for divergence and errors.

-> Move the code to create pickler and unpickler to internal zodbpickle
package as a preparatory step for that.

For the reference: this package is merely a wrapper around ogórek, not a
fork of any code like zodbpickle/py is.