Partial and machineless

### Changes include:
* `PostgresInterface` interface changed :),
* server now understands partial transactions and bills,
* in case of machine going offline server automatically sends tx after `SESSION_TIMEOUT `.


### New stuff:
* main coins sending logic gathered in `executeTx`, and 
* comments added to make it easier to read and understand,
* `summaryTx` renamed to `insertTx`, and now, in case of err, returns err, instead of list of txs,
* API now supports responding with 201/204 status codes for bills and txs (not yet enabled),
* `/send` endpoint now always returns `txId`,
* unused `getTxs` and `_getTxs` methods removed,
* `is_completed` is now always properly set/updated.

**Question:** Should we _bother_ machine with server-side errors (such as `insufficient funds`, etc), or should they be resolved silently on the server side?