We want to enable Subversion to use multiple concurrent sessions /
svn_fs_t instances during the data transfer phase of a commit transaction.
This is to support pipelined processing in http/2, for instance, and should
eliminate most of the latency penalty for commits with many change or added
files in them.

Keeping the tree transformation phase of the transaction strictly single-
threaded, only a small subset of operations needs to support concurrency:

* fs_apply_textdelta
* fs_apply_text
* fs_change_node_prop

In concurrent mode, these need to ensure mutally exclusive throughout the
system, i.e. across threads and processes.  The actual improvement comes
from the text change functions allowing concurrent writes to the streams
and delta processors they return.

Because concurrent mode needs a more expensive locking scheme and can't
use the DAG cache, we want this overhead to be optional.