Add debugging tools for ContextMemoryManager (#1407)

Add debugging tools for ContextMemoryManager (#1407)

This commit adds two configuration options that help debugging memory
issues with allocations in the ContextMemoryManager:

--enable/disable-valgrind: This flag enables/disables Valgrind
instrumentation of the ContextMemoryManager. Enabled by default for
debug builds if the Valgrind headers are available.

--enable/disable-context-memory-manager: This flag enables/disables the
use of the ContextMemoryManager. If the flag is disableda dummy
ContextMemoryManager is used that does single allocations instead of
chunks. The flag is enabled by default.