Add writemap et nometasync support

[HerveMARTIN]

For performance reason, it's often advised to add these two flags for mdb backend :
olcDbEnvFlags: writemap
olcDbEnvFlags: nometasync

I didn't find how to setup this with your module.
I'll look forward to propose a patch or a PR.

[bodgit]

Sorry, I've been on holiday for a couple of weeks, shouldn't be hard to add something to support this.

[bodgit]

I've added the following parameters, accesslog_envflags and data_envflags for supplying these flags. As I've been doing a lot of refactoring for a v2.0.0 release they are in that branch pending a release.

Thanks for the bug report!