[IceTV] Make the batch fetch size depend on available memory

Change the name of config variable config.plugins.icetv.batchsize to config.plugins.icetv.max_batchsize to better reflect its new meaning, and change the comment at its definition to reflect that.

Dynamically calculate the size of an IceTV fetch so that ~60MB remain during each batch fetch. The size of the batch is calculated by estimating the size of the channel data from and estimated per day/per channel data requirement and the amount of memory indicated in the MemAvailable entry in /proc/meminfo.

The batch size will be at least 1 channel and no more than config.plugins.icetv.max_batchsize channels. If max_batchsize is 0, the upper limit on the batch size will be all channels with IceTV EPG.

Bump version number.