Reduce replicator.retries_per_request value from 10 to 5

Previously an individual failed request would be tried 10 times in a row with
an exponential backoff starting at 0.25 seconds. So the intervals in seconds
would be:

   `0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, 128`

For a total of about 250 seconds (or about 4 minutes). This made sense before
the scheduling replicator because if a replication job had crashed in the
startup phase enough times it would not be retried anymore. With a scheduling
replicator, it makes more sense to stop the whole task, and let the scheduling
replicatgor retry later. `retries_per_request` then becomes something used
mainly for short intermettent network issues.

The new retry schedule is

   `0.25, 0.5, 1, 2, 4`

Or about 8 seconds.

An additional benefit when the job is stopped quicker, the user can find out
about the problem sooner from the _scheduler/docs and _scheduler/jobs status
endpoints and can rectify the problem. Otherwise a single request retrying for
4 minutes would be indicated there as the job is healthy and running.

Fixes apache#810

rel/overlay/etc/default.ini

@@ -370,7 +370,7 @@ connection_timeout = 30000
 ; Request timeout
 ;request_timeout = infinity
 ; If a request fails, the replicator will retry it up to N times.
-retries_per_request = 10
+retries_per_request = 5
 ; Use checkpoints
 ;use_checkpoints = true
 ; Checkpoint interval

src/couch_replicator/src/couch_replicator_api_wrap.hrl

@@ -21,7 +21,7 @@
     ],
     timeout,            % milliseconds
     ibrowse_options = [],
-    retries = 10,
+    retries = 5,
     wait = 250,         % milliseconds
     httpc_pool = nil,
     http_connections,

src/couch_replicator/src/couch_replicator_docs.erl

@@ -466,7 +466,7 @@ make_options(Props) ->
     DefBatchSize = config:get("replicator", "worker_batch_size", "500"),
     DefConns = config:get("replicator", "http_connections", "20"),
     DefTimeout = config:get("replicator", "connection_timeout", "30000"),
-    DefRetries = config:get("replicator", "retries_per_request", "10"),
+    DefRetries = config:get("replicator", "retries_per_request", "5"),
     UseCheckpoints = config:get("replicator", "use_checkpoints", "true"),
     DefCheckpointInterval = config:get("replicator", "checkpoint_interval",
         "30000"),