Support context in lock

[zacscoding]

Support context parameters when acquiring a lock.

Hello, we want to cancel or receive timeout error when trying to acquire a lock.
So, I checked whether the context is done or not for each step.

I think it would be better if zk.Conn's functions are based on context such as Create(ctx context.Context, ...), but currently not supported :(

[nemith]

I think this looks good, however I think locks should be in a different package and we should keep this around just for backwards compatibility until we can remove it in a v2 cut.

We should probably revisit https://github.com/go-zookeeper/recipes/issues/1 and #29

I am starting to think we should put these as sub packages to github.com/go-zookeeper/zk repository so we can avoid breakage between two different modules.

[codecov]

Codecov Report

Merging #67 (377f917) into master (8866d76) will increase coverage by 0.42%.
The diff coverage is 45.71%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   75.27%   75.69%   +0.42%     
==========================================
  Files           7        7              
  Lines        1189     1218      +29     
==========================================
+ Hits          895      922      +27     
- Misses        202      206       +4     
+ Partials       92       90       -2     

Impacted Files	Coverage Δ
lock.go	60.00% <45.71%> (-3.39%)	⬇️
constants.go	75.00% <0.00%> (ø)
conn.go	73.46% <0.00%> (+1.88%)	⬆️

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