[AutoTVM] Fix database APIs

[comaniac]

The database APIs designed in AutoTVM will throw exceptions with Redis database in Python 3. The main reason is that Python 3 package redis returns values in bytes format instead of str so it cannot be manipulated by builtin string methods like split.

The original code uses str(result) to convert bytes to str, but this is not the right way:

bval = b"some strings" # type(bval) -> <class 'byte'>
sval = str(bval) # type(sval) -> <class 'str'>
print(sval)
'b\'some strings\''

As can be seen, str simply invokes bytes.__str__() and results in unnecessary b\'...\' characters. The right way to do so is using decode method:

dval = bval.decode() # type(dval) -> <class 'str'>
print(dval)
'some strings'

This PR fixes the issue and creates a new unit test to cover filer method which wasn't covered before. With the changes, the unit test passes for both DummyDatabase and RedisDatabase locally.

One remain issue could be the lack of unit tests for Redis database, but this requires changes of CI environment.

[comaniac]

@icemelon9 please help review and merge.

[comaniac]

@wweic thanks for review. Please see my comments and new commits.

[icemelon]

Thanks @comaniac @wweic this is now merged