Guarantee word-aligned writes

[philips77]

This PR fixed #358. I still need to know why it is important before merging it.

[philips77]

@SuhaibI Could you please review?

[SuhaibI]

@philips77 Really sorry I missed your pings, and this slipped my mind as we rolled the change into a private fork.

This change has no effect in the majority of cases I've seen with iOS because the nRF5 SDK logic negotiates an MTU that is word aligned - the nominal case with iOS is: iOS sends 185 as max MTU, device saves 183 as desired MTU, which ends up being word aligned.
I've seen an issue in the following rare case:

• iOS rarely negotiates a much higher MTU (255 bytes)
• Device sets maximum MTU to 255 bytes
• On the next connection iOS now reports a max of 185 bytes - this becomes the negotiated MTU since it's the minimum of 185 and 255
• Writes are no longer word aligned
  A potential improvement to the nRF5 SDK would be to stop initiating a MTU re-negotiation and depend on the client to make word-aligned writes. This could save some time during connection.

[philips77]

Ok, let's merge it. I'll release it with the next version.