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ARROW-5639: [Java] Remove floating point computation from getOffsetBufferValueCapacity

Some getOffsetBufferValueCapacity methods use floating point computation to calculate the capacity, which is not necessary.

(int) ((offsetBuffer.capacity() * 1.0) / OFFSET_WIDTH);

It is interesting to note that JIT cannot optimize away the floating point computations:

So this has performance penalty:
Before:
VariableWidthVectorBenchmarks.getValueCapacity avgt 5 6.570 ± 0.004 ns/op

After:
VariableWidthVectorBenchmarks.getValueCapacity avgt 5 5.787 ± 0.575 ns/op

Author: liyafan82 <fan_li_ya@foxmail.com>

Closes #4602 from liyafan82/fly_0618_mult and squashes the following commits:

dd905f3d6 <liyafan82>  Remove more floating point computations
dee0fcf4e <liyafan82>  Remove floating point computation from BaseVariableWidthVector#getOffsetBufferValueCapacity