Fixed a compiler warning from clang.
Splitted resource and resource usage checkers.

Review: https://reviews.apache.org/r/28618
Added basic DiskInfo check in master.

Review: https://reviews.apache.org/r/28626
Added tests for basic DiskInfo checker.

Review: https://reviews.apache.org/r/28627
Added duplicated persistence id check in ResourceChecker.

Review: https://reviews.apache.org/r/28664
Renamed task and offer visitors to validators.

Review: https://reviews.apache.org/r/28684
Moved repeated labels to Labels message.

We suggested to change hooks into manipulating task labels only. With
that in mind and based on BenH's suggestion originally, this patch set
wraps the repeated Labels field in a message when can be used without
referring to repeated field types.

Review: https://reviews.apache.org/r/28574
Changed task to use Labels.

Review: https://reviews.apache.org/r/28575
Changed HTTP task modeling to use Labels.

Review: https://reviews.apache.org/r/28576
Changed tests to use Labels.

Review: https://reviews.apache.org/r/28577
Disable building static libraries

Shrinks the size resulting from a 'make install' considerably.

On a build with the options:
../configure --enable-optimize build

du -h of the install directory gives:
52MB with patch
113MB without patch

Review: https://reviews.apache.org/r/28082