Use Boehm-Demers-Weiser Garbage Collector. (#647)

* Use Boehm-Demers-Weiser Garbage Collector.

From their own documentation:

> The Boehm-Demers-Weiser conservative garbage collector can be used as a
> garbage collecting replacement for C malloc or C++ new. It allows you to
> allocate memory basically as you normally would, without explicitly
> deallocating memory that is no longer useful. The collector automatically
> recycles memory when it determines that it can no longer be otherwise
> accessed.

In pgcopydb C code base it's been hard to maintain proper malloc/free
concerns while also implementing proper error-handling, which is a common
problem when writing C code. In order to ease the maintenance of the code
and reduce production hazards, the best way to proceed looks like automating
the memory management altogether by using a Garbage Collector.

* Revert removal of regree() calls.

* Get rid of LinesBuffer.ownsBuffer.

Now that libgc is handling the memory for us, we don't need to keep track of
the ownership of the buffer anymore.