rxnorm linker doesn't work with multiprocessing?

Hi, I'm getting an error trying to run `nlp.pipe` with `n_processes > 1`, I think because the pickling that `multiprocessing` does under the hood interacts poorly with `nmslib.dist.FloatIndex`, which the rxnorm entity linker requires and does not seem picklable.

Minimal code:

```
import spacy
import scispacy
from scispacy.linking import EntityLinker

TEXTS = ["Hello! This is document 1.", "And here's doc 2."]

if __name__ == '__main__':
nlp = spacy.load("en_core_sci_sm")
nlp.add_pipe("scispacy_linker", config={"resolve_abbreviations": True,
"linker_name": "rxnorm"})
for doc in nlp.pipe(TEXTS, n_process=2):
print(doc)
```
Running with Python 3.8.5 gives me:
```
Traceback (most recent call last):
File "./mwerror.py", line 13, in
for doc in nlp.pipe(TEXTS, n_process=2):
File ".../python3.8/site-packages/spacy/language.py", line 1479, in pipe
for doc in docs:
File ".../python3.8/site-packages/spacy/language.py", line 1515, in _multiprocessing_pipe
proc.start()
File ".../python3.8/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File ".../python3.8/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File ".../python3.8/multiprocessing/context.py", line 284, in _Popen
return Popen(process_obj)
File ".../python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
super().__init__(process_obj)
File ".../python3.8/multiprocessing/popen_fork.py", line 19, in __init__
self._launch(process_obj)
File ".../python3.8/multiprocessing/popen_spawn_posix.py", line 47, in _launch
reduction.dump(process_obj, fp)
File ".../python3.8/multiprocessing/reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'nmslib.dist.FloatIndex' object
```
Note I don't get an error with `n_process=1`, presumably because `multiprocessing` is not invoked.

I also do not get this error if I don't include the linker pipe (i.e. comment out the `add_pipe()` line above).

Thanks! This lib is great!