Share a singleton engine to share between requests

Each WSGI worker thread can get engines via:
`Repository.create_engine(db_url)`

Using apache benchmark and the sample data from
https://github.com/GeoNode/gisdata/tree/master/gisdata/metadata I saw a
reduced execution time of 18%.

Here's the raw data:

$ ab -n 120 -c 15 -H "Accept-Encoding: gzip,deflate" "http://127.0.0.1:8080/?service=CSW&version=2.0.2&&mode=opensearch&servi
ce=CSW&version=2.0.2&request=GetRecords&typenames=csw:Record&resulttype=results&elementsetname=brief"
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient).....done

Server Software:        gunicorn/18.0
Server Hostname:        127.0.0.1
Server Port:            8080

Document Path:          /?service=CSW&version=2.0.2&&mode=opensearch&service=CSW&version=2.0.2&request=GetRecords&typenames=csw:Record&resulttype=resu
lts&elementsetname=brief
Document Length:        15387 bytes

Concurrency Level:      15
Time taken for tests:   10.038 seconds
Complete requests:      120
Failed requests:        0
Write errors:           0
Total transferred:      1866480 bytes
HTML transferred:       1846440 bytes
Requests per second:    11.95 [#/sec] (mean)
Time per request:       1254.740 [ms] (mean)
Time per request:       83.649 [ms] (mean, across all concurrent requests)
Transfer rate:          181.58 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.1      0       1
Processing:    89 1182 234.9   1254    1298
Waiting:       88 1182 234.9   1254    1298
Total:         90 1183 234.8   1254    1298

Percentage of the requests served within a certain time (ms)
  50%   1254
  66%   1256
  75%   1260
  80%   1262
  90%   1265
  95%   1270
  98%   1273
  99%   1298
 100%   1298 (longest request)

$ ab -n 120 -c 15 -H "Accept-Encoding: gzip,deflate" "http://127.0.0.1:8080/?service=CSW&version=2.0.2&&mode=opensearch&servi
ce=CSW&version=2.0.2&request=GetRecords&typenames=csw:Record&resulttype=results&elementsetname=brief"
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient).....done

Server Software:        gunicorn/18.0
Server Hostname:        127.0.0.1
Server Port:            8080

Document Path:          /?service=CSW&version=2.0.2&&mode=opensearch&service=CSW&version=2.0.2&request=GetRecords&typenames=csw:Record&resulttype=resu
lts&elementsetname=brief
Document Length:        15387 bytes

Concurrency Level:      15
Time taken for tests:   8.176 seconds
Complete requests:      120
Failed requests:        0
Write errors:           0
Total transferred:      1866480 bytes
HTML transferred:       1846440 bytes
Requests per second:    14.68 [#/sec] (mean)
Time per request:       1021.957 [ms] (mean)
Time per request:       68.130 [ms] (mean, across all concurrent requests)
Transfer rate:          222.95 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.1      0       0
Processing:   212  952 151.0    983    1121
Waiting:      212  951 151.0    983    1121
Total:        213  952 150.9    983    1122

Percentage of the requests served within a certain time (ms)
  50%    983
  66%    988
  75%    994
  80%    999
  90%   1048
  95%   1060
  98%   1090
  99%   1113
 100%   1122 (longest request)
Merge pull request #211 from ahinz/fix/130/shared-database-engine

Share a singleton engine to share between requests