Fix uninitialized variable warning:

../src/conn/conn_cache_pool.c: In function '__wt_cache_pool_server':
../src/conn/conn_cache_pool.c:394:38: error: 'adjusted' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if (cp->currently_used <= cp->size && !adjusted)
                                      ^
../src/conn/conn_cache_pool.c:460:11: note: 'adjusted' was declared here
  uint64_t adjusted, reserved, read_pressure;
           ^
cc1: all warnings being treated as errors