refactoring: extract wal writer, rename replication.[hc] -> relay.[hc]
Extract the write ahead log into its own C file, rename
replication.[hc] to relay.[hc]
Cleanup headers.
No semantical code chagnes.
wal: begin changes necessary to switch to the common bus

Implement a generic inter-cord communication bus and use it
in separating network I/O and transaction processor.

Split all request processing work into network thread part
and transaction processor thread part, and make these
parts multi-threading safe.

Generalize fiber pool into an own data structure and API.

Misc cleanups.

Conflicts:
	src/iproto.cc

Conflicts:
	src/box/iproto.cc

Conflicts:
	src/iproto.cc

When selecting the next buffer to flush, take into account not
just its size, but how much of it has already been flushed.
This saves a couple syscalls per connection per event loop.

Refactor iobuf and extract ibuf/obuf into small allocators
library.
Fix leaking obuf_destroy() (some slabs could be leaked by
destroy).

Introduce rename to follow the style of small allocators library.

Display how much memory is used by a fiber, and the total size.

port_buf has similar purpose but uses mempool instead of realloc()

- fix a coding bug/regression in fiber.cc with wrong
  check of result of the dynamic cast
- style fixes
- fix initialization of type objects
  when some members of inheritance hierarchy were
  excluded from type hierarchy

Conflicts:
	src/box/recovery.cc
	src/box/xrow.cc