Make read and write threadsafe

[JonasIsensee]

This PR aims to make reading and writing independent files on multiple threads possible
by removing unnecessary global states and protecting necessary ones with ReentrantLocks.

Somehow this should have tests..

[codecov]

Codecov Report

Merging #230 into master will increase coverage by 0.80%.
The diff coverage is 98.36%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   89.11%   89.91%   +0.80%     
==========================================
  Files          21       21              
  Lines        2241     2410     +169     
==========================================
+ Hits         1997     2167     +170     
+ Misses        244      243       -1     

Impacted Files	Coverage Δ
src/JLD2.jl	96.25% <97.43%> (+0.44%)	⬆️
src/datasets.jl	95.79% <100.00%> (+0.30%)	⬆️
src/mmapio.jl	81.88% <100.00%> (+1.41%)	⬆️
src/stdlib.jl	96.66% <100.00%> (+0.51%)	⬆️
src/misc.jl	100.00% <0.00%> (ø)
src/attributes.jl	100.00% <0.00%> (ø)
src/backwards_compatibility.jl	100.00% <0.00%> (ø)
src/global_heaps.jl	97.36% <0.00%> (+0.10%)	⬆️
src/dataio.jl	97.31% <0.00%> (+0.21%)	⬆️
... and 15 more

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