misc: iproto, iobuf cleanups

- encapsulate iproto configuration into iproto module API
- iobuf: change the module to be thread-safe.
- iobuf: rename obuf_ensure() to obuf_reserve() as is conventional
  with the reset of API.
- iobuf: change ibuf to use slab cache directly, bypassing the region allocator
- iobuf: consistently use int for obuf/obuf_svp iov pos and obuf_iovcnt
- iobuf: remove iobuf_flush
- iobuf: add iobuf pin/unpin API in preparation for iproto-in-a-thread
- iobuf: don't trust iov_len as an end marker in obuf_reset()
  There is a chance (although currently it's never used) that one of the
  vectors is allocated, but not used, and the next vector of a larger size
  is used. In that case reset loop may stop prematurely.