Tell valgrind about memory pools
Valgrind has a set of macros to tell it about custom memory allocators (see http://valgrind.org/docs/manual/mc-manual.html#mc-manual.mempools).
I tried to use them to tell valgrind about our fix-sized pool, but the results did not meet my expectation.
See the branch mc/valgrind, with two different attempts (commits 631a3cf0 and d784ffd7).
See file mempool.cc for a description of expected behaviors.
NB: I only tried valgrind 3.10, the current package in debian stable (jessie). valgrind 3.12 introduces macros for two-level pools (see valgrind doc), which may be more appropriate to our use-case.