Optimize ra_svn's protocol item parser.  Eliminate as much overhead
as possible for the most commonly used types.

As part of this, we also limit the length of WORD elements in the
protocol.  The set of possible words are pre-defined by the protocol
and the longest one today has 16 chars.  Limiting them to 31 chars
should never be a problem for us but facilitates the use of fixed-
size buffers.

* subversion/libsvn_ra_svn/marshal.c
  (MAX_WORD_LENGTH): define new constant
  (read_string): short strings can usually be copied directly from the
                 connection's read buffer
  (read_item): eliminate multiplication from the int64 overflow check;
               use a fixed buffer for words;
               manually create and populate item arrays

* subversion/libsvn_ra_svn/protocol
  (4.1): new section documenting that words are limited to 31 chars


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1485851 13f79535-47bb-0310-9956-ffa450edef68