Fix read workload regression.
Attempt at fixing compilation issue #3369. Moved struct hash declaration from docid.h to basictypes.h.
Included <functional> instead of hacking around it with a manual std::hash declaration
Fixes to handle infinite streams
Fixes review feedback
Fixes `r.map` in the Javascript driver
Use jemalloc instead of tcmalloc by default

Closes #2279
Review 2353 by danielmewes
Removed file_account_t argument from index_write.
fix deadlock on errors in the backup scripts
Update mk/README.md
Use a common file account for block writes in the merger serializer to improve write I/O efficiency.
Merge branch 'next' into daniel_3377
Merge branch 'mlucy_3354' into next
Merge branch 'daniel_3377' into next
Disable OS' read-ahead when using buffered i/o on Linux.
Disable read-ahead on OS X as well.
Revert "Disable read-ahead on OS X as well."

This reverts commit 3cb0b7fa57cb7b24fd38c63c269a025d4719fd54.
Disable read-ahead on OS X as well.
Merge branch 'daniel_fadvise' into next
Make buffered I/O the default. --no-direct-io is now deprecated, --direct-io introduced instead.