Implement connection parameter update mechanisms

[jonas-schievink]

There are 3 mechanisms for the slave to request specific connection parameters:

• Link Layer Connection Parameters Request Procedure
  • Requires support for LLCPDUs LL_REJECT_IND_EXT, LL_CONNECTION_PARAM_REQ, LL_CONNECTION_PARAM_RSP
  • Optional feature: Must indicate support by setting feature flags for EXTENDED_REJECT_INDICATION and CONN_PARAM_REQ
• L2CAP Signaling Connection Parameters Request Procedure
  • Simpler version of the above
  • Mandatory
• GAP Characteristic Peripheral Preferred Connection Parameters
  • Simple if we had build.rs-based service declarations #72, now it's pretty complex
  • Optional characteristic, must be respected by Central, can be put in adv. data too

I've started to prototype the LLCP procedure, but it's not very simple to do.

Since we'll have to implement the mandatory L2CAP signaling channel and the GAP service anyways, I think going for the L2CAP solution is simplest (and results in the least code bloat).

cc @perlindgren