Redis reversed history fix when meta does not exist

[FZambia]

Fixes centrifugal/centrifugo#732

I.e. fixes this error:

Lua redis lib command arguments must be strings or integers script: c9735f3dd5ac99ee9c4e59d9f5ac297b036f9443, on @user_script:25.

Also contains Redis broker LUA scripts refactoring like we did for presence recently.

Also, save one Redis call in history get scripts by using hmget instead of two hget calls.

[codecov]

Codecov Report

Merging #328 (2462188) into master (816d5db) will decrease coverage by 0.14%.
Report is 5 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
- Coverage   83.85%   83.72%   -0.14%     
==========================================
  Files          38       38              
  Lines        7472     7472              
==========================================
- Hits         6266     6256      -10     
- Misses        893      902       +9     
- Partials      313      314       +1     

Files	Coverage Δ
broker_redis.go	73.05% <ø> (-0.70%)	⬇️

... and 3 files with indirect coverage changes

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