In the current implementation, as soon as one trajectory is frozen it is not possible to use new landmarks anymore.
In this PR a suggest a change, that allows introducing new landmarks also in the presence of a frozen trajectory. This is done with the following steps:
- Store set of frozen landmarks
- Landmarks that are loaded from a pbstream, with the flag ` load_frozen_state=true` are inserted into the set of frozen landmarks via `setLandmarkPose` function
- frozen landmarks are set constant in the optimization problem
- new landmarks can be introduced and will be optimized in the optimization problem

Motivated by and closes #1470

* Moved value converter include out of internal folder.

* Ran clang-format.

Eigen::Matrix (or Eigen::Array) with custom scalar types is
not intended for arbitrary structs:
https://eigen.tuxfamily.org/dox/TopicCustomizing_CustomScalar.html

This implements a simple class as a replacement.

- use `pose_queue_duration_` instead of hardcoded "0.001"
  (cartographer_ros sets this to 0.001, probably that's why this
  wasn't noticed)
- queue_delta is in seconds, log message said milliseconds

The introduced changes are not compatible with CMake < 3.6 if the
FindProtobuf.cmake bundled with CMake is used.
Since our minimum required version is 3.2, I consider this to be a bug.

- camel-case variables names weren't used before 3.6
  (upper-case is lowered in CMake > 3.6 for backwards compatibility)
- the `CONFIG` search for the Windows build logs a warning even if the
`REQUIRED` search succeeds, should be quiet

Protobuf 3.5 would require a Bazel flag
bazel build --incompatible_package_name_is_a_function=false
for Bazel 0.21 because its bazel syntax is old.
Rather than building with this flag, let's just update
the protobuf version.

Also, fix lint.

std::string::operator+ is very inefficient

Previously, two config values were spelled "bandwith".
The assumption is that these values are not used in other
repositories so the rename is harmless.

This silently worked when it was written, but in order 
to check out a specific commit we cannot have a shallow
clone of the master.

The update of the prometheus library requires some fixes in the test.

This PR makes libcartographer build on Windows (including tests). Abseil was bumped to avoid a MSVC compiler bug.

I have observed two tests failing:
`MapBuilderTestByGridType/MapBuilderTestByGridType.GlobalSlam2D/1` and `MapBuilderTestByGridType/MapBuilderTestByGridType.LocalizationOnFrozenTrajectory2D/1`.

Was hardcoded to 3 before, but it makes sense to make this a parameter.

* Switch to new bazel workspace rules

* Add another override

Moving next Open House to January.

Since the constraint builder metrics are ever-increasing, it makes sense
to add gauges that monitor the current counts of constraints in the pose
graph. The main advantages are that they also take trimming into account
and distinguish constraints within or across trajectories.

Got linker error for Abseil in the build env due to imprecise library order.
Worked fine on the dev machine though.
Maybe helps someone.

https://pastebin.com/raw/AEKPBt1n

Allows to build Docker images with Clang.

Clang with thread safety does not compile because
num_nodes_since_last_loop_closure_
is modified without holding a mutex.
This fixes it.

This is necessary because otherwise derived classes
are potentially deleted incompletely, using the destructor
of the interface.

This change improves the portability of Cartographer.

https://cmake.org/cmake/help/v3.2/variable/CMAKE_LANG_COMPILER_ID.html

Noticed ToFlatIndex during profiling. Inline a couple of other short functions as well.

Changes the public map builder API, but is required to not break the ROS API.
See also https://github.com/googlecartographer/cartographer_ros/pull/1014 and #1422