Eliminate race conditions

[lionelains]

When using threads for timer or libserialcpp SPI (that is internally using threads also), there may be additional race conditions that are not usually hit if running a mainloop (eg: on Raritan framework).

Serial read/write coud can be run simultaneously and race conditions must be handled.
Timer elapsing are implemented using threads when not running on the Raritan mainloop and the actions for the code executed in timers should be protected using mutexes.

[lionelains]

We should first add a mutex SPI to be able to run either with Raritan framework mutexes of C++11 mutexes.

[lionelains]

One race condition here:
CLibEzspMain lib_main is instanciated and could start to invoke observer callbacks, but the callbacks themselves are registered a bit afterwards.

[lionelains]

In ezsp-dongle.cpp:
Reset command is sent here but callbacks for received frames is only registered afterwards.

[lionelains]

First race condition should be fixed by f7d20b7

Second race condition should be fixed by 31a2e16

[lionelains]

Fixes done on branch https://github.com/Legrandgroup/libezsp/tree/thread-safety are part of solving this.

[lionelains]

Implemented in PR #76