Use FlatBuffers for cflamegraph files

[kornilova203]

Snapshot version of plugin implements compressed version of flamegraph format.
But the new format is not binary therefore it is not optimal for saving disk space.
Also custom parser is implemented for the format and it is bug prone.

I suggest keeping structure of cflamegraph files but using FlatBuffers for serialization.

cflamegraph format will contain the same amount of information as regular flamegraph, therefore it will not be possible to directly use it for client-server communication #15, but it can be easily converted to more complex data.

I considered making cflamegraph format to be also client-server format, but then it would contain redundant information (that is set to speed up visualization) also client-server format may be changed and I do not want to worry about support for existing files.