Cast const string into no-const.

This fixes "deprecated conversion from string constant to ‘char*’ [-Werror=write-strings]" warning

This closes bug #1499
Merge pull request #1500 into next

Closes #1500

Closes #1499

Closes #1484
Added a sync_t write operation
Completed the implementation of a sync term in the server.
Added API docs and Python/JavaScript support for sync.
Added a simple polyglot test for sync
Small doc improvements.
Merge branch 'daniel_sync_term' into daniel_slow_flushes
Partially integrated sync into the sub-patch code. It is broken however, because we need to resolve DURABILITY_REQUIREMENT_DEFAULT.