Allow rocksdb to accept read_only option in order to share tables among workers

[wbarnha]

Closes #330, I've had success by running multiple Faust apps with the same data directory for RocksDB

app = faust.App("reuse-this-app-name")
app.GlobalTable('generic-table-name', options={'read_only': True})

I'd like to write some test cases/examples before merging this in order to understand how this works better. If this functionality is not intended, feel free to close this immediately.

[codecov-commenter]

Codecov Report

Merging #331 (d4f03c2) into master (9eb770a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #331   +/-   ##
=======================================
  Coverage   94.42%   94.42%           
=======================================
  Files         100      100           
  Lines       10903    10905    +2     
  Branches     1529     1529           
=======================================
+ Hits        10295    10297    +2     
  Misses        533      533           
  Partials       75       75           

Impacted Files	Coverage Δ
faust/stores/rocksdb.py	89.45% <100.00%> (+0.06%)	⬆️

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[patkivikram]

Is this tested with https://github.com/faust-streaming/python-rocksdb? Thats the official rocksdb binding we are using for this project

[wbarnha]

I've been using https://github.com/NightTsarina/python-rocksdb with Faust. I made a PR for that update in faust-streaming/python-rocksdb#1 so that we can synchronize progress on all forks. Feel free to merge that whenever you feel comfortable.

I just tested this PR with the current release of https://github.com/faust-streaming/python-rocksdb and I can confirm everything works properly. Before using the changes in this PR, I got errors such as IO error: lock [DB path]/LOCK: Resource temporarily unavailable from RocksDB.

It'd be nice if I could make the read_only option more accessible.