optimize database sequence compression

[westram]

Problems:

1. when copying an alignment, master sequence (MS) compression is NOT copied ⇒ destination alignment uses more space than source alignment
2. when sequence data gets modified, MS compression is dropped

Solutions:

1. copy MS information when copying alignment
2. reuse previously used MS when compressing changed sequence

Special case:

• when inserting/deleting columns, the MS could be adapted (=perform insert/delete there as well)

Possible optimization:

• currently optimize uses the following steps:
  1. re-compress sequences w/o using MS
  2. delete old and create new MS
  3. re-compress sequences using MS
• step 1. could be skipped by using 2 sets of MS and 2 separate 'compression mode' flags (currently always GB_COMPRESSION_SEQUENCE). optimize then would perform as:
  1. create new MS
  2. re-compress sequences using old MS to decompress and new MS to compress
  3. delete old MS