logserver: Support for query with block number and so on

[kvinwang]

This PR implements the feature requests from @Leechael.

• Setting the start sequence relative-to the next_sequence.
  The filter argument from now can be negative, which means counting back from the end like the list indexing in Python.
  For example given next_sequence = 100, when from = -2 means from = 98.
• Filtering records with block number if given.
  A new argument blockNumber is added to the GetLog action.
• Adding an action named GetInfo which returns infomations as shown below:

  {
    "currentNumberOfRecords": 4,
    "estimatedCurrentSize": 675,
    "memoryCapacity": 1048576,
    "memoryUsage": 22519,
    "nextSequence": 4,
    "programVersion": [0, 1, 0]
  }

[Leechael]

lgtm. I will do SDK compatible check later.