Repeating database writes

[EinKrebs]

Which problem is this PR solving?

• 2nd and partly 1st point in #75

Short description of the changes

• Now batch writing is done by WriteWorker(naming is not final, please help me with it). It uses exponentiall backoff algorithm: in case of error writing to database it keeps trying, with icreasing duration between attempts;
• WriteWorkers are created by WriteWorkerPool. If there's too many spans being ritten, the oldest WriteWorker is being stopped, and its batch is lost;
• SpanWriter creates WriterWorkerPool and passes batches to it;
• Also all info needed for writing spans is held in WriteParams struct, which is shared between all WriteWorkers.

I'm planning to use heap as collection for WriteWorkers, which will make WriteWorkesPool's work faster. Done

[EinKrebs]

I'm looking for advices about better package structure.

[EinKrebs]

I also tested it with pushing 1e8 traces via tracegen:

SELECT count()
FROM jaeger_index 
WHERE service = 'tracegen'

┌──count()─┐
│ 98139237 │
└──────────┘

which is more than in current version(only 57026426 spans)